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;
+}