nrf: Fix _start() build issue with CMSIS 5.9.0.

The `_start` function prototype is now declared as no-return, so `main()`
can't return.

To fix this, `main()` is replaced with `_start`.

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

ports/nrf/drivers/secureboot/secureboot_main.c

@@ -176,7 +176,7 @@ static void jump_to_non_secure(void)
     }
 }
 
-int main(void) {
+void _start(void) {
     configure_flash();
     configure_ram();
     configure_peripherals();
@@ -186,9 +186,4 @@ int main(void) {
     while (1) {
 	;
     }
-
-    return 0;
 }
-
-void _start(void) {main();}
-

ports/nrf/main.c

@@ -107,7 +107,7 @@ void do_str(const char *src, mp_parse_input_kind_t input_kind) {
 extern uint32_t _heap_start;
 extern uint32_t _heap_end;
 
-int main(int argc, char **argv) {
+void NORETURN _start(void) {
     // Hook for a board to run code at start up, for example check if a
     // bootloader should be entered instead of the main application.
     MICROPY_BOARD_STARTUP();
@@ -299,8 +299,6 @@ soft_reset:
     #endif
 
     goto soft_reset;
-
-    return 0;
 }
 
 #if !MICROPY_VFS
@@ -369,7 +367,3 @@ void MP_WEAK __assert_func(const char *file, int line, const char *func, const c
     printf("Assertion '%s' failed, at file %s:%d\n", expr, file, line);
     __fatal_error("Assertion failed");
 }
-
-void _start(void) {
-    main(0, NULL);
-}