stm32/main: Rename main to stm32_main and pass through first argument.

The main() function has a predefined type in C which is not so useful for
embedded contexts.  This patch renames main() to stm32_main() so we can
define our own type signature for this function.  The type signature is
defined to have a single argument which is the "reset_mode" and is passed
through as r0 from Reset_Handler.  This allows, for example, a bootloader
to pass through information into the main application.

ports/stm32/main.c

@@ -413,7 +413,7 @@ STATIC uint update_reset_mode(uint reset_mode) {
     return reset_mode;
 }
 
-int main(void) {
+void stm32_main(uint32_t reset_mode) {
     // TODO disable JTAG
 
     /* STM32F4xx HAL library initialization:
@@ -488,7 +488,7 @@ soft_reset:
 #endif
     led_state(3, 0);
     led_state(4, 0);
-    uint reset_mode = update_reset_mode(1);
+    reset_mode = update_reset_mode(1);
 
     // Python threading init
     #if MICROPY_PY_THREAD

ports/stm32/resethandler.s

@@ -33,6 +33,9 @@
     .type Reset_Handler, %function
 
 Reset_Handler:
+    /* Save the first argument to pass through to stm32_main */
+    mov  r4, r0
+
     /* Load the stack pointer */
     ldr  sp, =_estack
 
@@ -61,6 +64,7 @@ Reset_Handler:
 
     /* Initialise the system and jump to the main code */
     bl   SystemInit
-    b    main
+    mov  r0, r4
+    b    stm32_main
 
     .size Reset_Handler, .-Reset_Handler