armcm_boot: Add generic code for early board init on armcm machines

Add basic ARM Cortex-M C init code and build linker scripts to
src/generic/ code.  This can be used to simplify the various ARM board
code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/armcm_boot.c

@@ -0,0 +1,73 @@
+// ARM Cortex-M vector table and initial bootup handling
+//
+// Copyright (C) 2019  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "armcm_boot.h" // DECL_ARMCM_IRQ
+#include "board/internal.h" // SystemInit
+
+// Symbols created by armcm_boot.lds.S linker script
+extern uint32_t _data_start, _data_end, _data_flash;
+extern uint32_t _bss_start, _bss_end, _stack_start;
+
+
+/****************************************************************
+ * Basic interrupt handlers
+ ****************************************************************/
+
+// Initial code entry point - invoked by the processor after a reset
+void
+ResetHandler(void)
+{
+    // Copy global variables from flash to ram
+    uint32_t count = (&_data_end - &_data_start) * 4;
+    __builtin_memcpy(&_data_start, &_data_flash, count);
+
+    // Clear the bss segment
+    __builtin_memset(&_bss_start, 0, (&_bss_end - &_bss_start) * 4);
+
+    barrier();
+
+    // Initializing the C library isn't needed...
+    //__libc_init_array();
+
+    // Initialize the machine
+    SystemInit();
+
+    // Run the main code
+    extern int main(void);
+    main();
+
+    // The main() call should not return
+    for (;;)
+        ;
+}
+DECL_ARMCM_IRQ(ResetHandler, -15);
+
+// Code called for any undefined interrupts
+void
+DefaultHandler(void)
+{
+    for (;;)
+        ;
+}
+
+
+/****************************************************************
+ * Dynamic memory range
+ ****************************************************************/
+
+// Return the start of memory available for dynamic allocations
+void *
+dynmem_start(void)
+{
+    return &_bss_end;
+}
+
+// Return the end of memory available for dynamic allocations
+void *
+dynmem_end(void)
+{
+    return &_stack_start;
+}

src/generic/armcm_boot.h

@@ -0,0 +1,20 @@
+#ifndef __GENERIC_ARMCM_BOOT_H
+#define __GENERIC_ARMCM_BOOT_H
+
+#include "ctr.h" // DECL_CTR_INT
+
+// Declare an IRQ handler
+#define DECL_ARMCM_IRQ(FUNC, NUM)                               \
+    DECL_CTR_INT("DECL_ARMCM_IRQ " __stringify(FUNC), (NUM))
+
+// Statically declare an IRQ handler and run-time enable it
+#define armcm_enable_irq(FUNC, NUM, PRIORITY) do {      \
+        DECL_ARMCM_IRQ(FUNC, (NUM));                    \
+        NVIC_SetPriority((NUM), (PRIORITY));            \
+        NVIC_EnableIRQ((NUM));                          \
+    } while (0)
+
+// Vectors created by scripts/buildcommands.py from DECL_ARMCM_IRQ commands
+extern const void * const VectorTable[];
+
+#endif // armcm_boot.h

src/generic/armcm_boot.lds.S

@@ -0,0 +1,63 @@
+// Generic ARM Cortex-M linker script
+//
+// Copyright (C) 2019  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "autoconf.h" // CONFIG_FLASH_START
+
+OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
+OUTPUT_ARCH(arm)
+
+MEMORY
+{
+  rom (rx) : ORIGIN = CONFIG_FLASH_START , LENGTH = CONFIG_FLASH_SIZE
+  ram (rwx) : ORIGIN = CONFIG_RAM_START , LENGTH = CONFIG_RAM_SIZE
+}
+
+SECTIONS
+{
+    .text : {
+        . = ALIGN(4);
+        KEEP(*(.vector_table))
+        *(.text .text.*)
+        *(.rodata .rodata*)
+    } > rom
+
+    . = ALIGN(4);
+    _data_flash = .;
+
+    .data : AT (_data_flash)
+    {
+        . = ALIGN(4);
+        _data_start = .;
+        *(.ramfunc .ramfunc.*);
+        *(.data .data.*);
+        . = ALIGN(4);
+        _data_end = .;
+    } > ram
+
+    .bss (NOLOAD) :
+    {
+        . = ALIGN(4);
+        _bss_start = .;
+        *(.bss .bss.*)
+        *(COMMON)
+        . = ALIGN(4);
+        _bss_end = .;
+    } > ram
+
+    _stack_start = CONFIG_RAM_START + CONFIG_RAM_SIZE - CONFIG_STACK_SIZE ;
+    .stack _stack_start (NOLOAD) :
+    {
+        . = . + CONFIG_STACK_SIZE;
+        _stack_end = .;
+    } > ram
+
+    /DISCARD/ : {
+        // The .init/.fini sections are used by __libc_init_array(), but
+        // that isn't needed so no need to include them in the binary.
+        *(.init)
+        *(.fini)
+    }
+}

src/generic/armcm_timer.c

@@ -5,6 +5,7 @@
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
 #include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "armcm_boot.h" // DECL_ARMCM_IRQ
 #include "board/internal.h" // SysTick
 #include "board/irq.h" // irq_disable
 #include "board/misc.h" // timer_from_us
@@ -160,6 +161,7 @@ SysTick_Handler(void)
     timer_set_diff(diff);
     irq_enable();
 }
+DECL_ARMCM_IRQ(SysTick_Handler, SysTick_IRQn);
 
 // Make sure timer_repeat_until doesn't wrap 32bit comparisons
 void