armcm_boot: Use armcm_main() instead of main() to start board code

The main() function has a special meaning to gcc and using it can
result in different code generation.  Use armcm_main() to avoid that.

Also, invoke SystemInit() from the board specific armcm_main() code.
This gives the board code more control over board initialization.

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

src/generic/armcm_boot.h

@@ -3,6 +3,8 @@
 
 #include "ctr.h" // DECL_CTR_INT
 
+void armcm_main(void);
+
 // Declare an IRQ handler
 #define DECL_ARMCM_IRQ(FUNC, NUM)                                       \
     DECL_CTR_INT("DECL_ARMCM_IRQ " __stringify(FUNC), 1, CTR_INT(NUM))