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.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