nrf/main: Add startup and early-init board hook macros.

Following stm32 for naming and location of the hooks.  Also allow a board
to provide custom .c files to implement code for these hooks.

ports/nrf/Makefile

@@ -274,6 +274,7 @@ SRC_C += \
 	drivers/ticker.c \
 	drivers/bluetooth/ble_drv.c \
 	drivers/bluetooth/ble_uart.c \
+	$(wildcard $(BOARD_DIR)/*.c) \
 
 ifeq ($(MCU_SUB_VARIANT), nrf52840)
 

ports/nrf/main.c

@@ -107,9 +107,14 @@ extern uint32_t _heap_start;
 extern uint32_t _heap_end;
 
 int main(int argc, char **argv) {
+    // 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();
 
+    MICROPY_BOARD_EARLY_INIT();
 
 soft_reset:
+
     #if MICROPY_PY_TIME_TICKS
     rtc1_init_time_ticks();
     #endif

ports/nrf/mpconfigport.h

@@ -366,3 +366,11 @@ extern const struct _mp_obj_module_t music_module;
 #ifndef MP_NEED_LOG2
 #define MP_NEED_LOG2                (1)
 #endif
+
+#ifndef MICROPY_BOARD_STARTUP
+#define MICROPY_BOARD_STARTUP()
+#endif
+
+#ifndef MICROPY_BOARD_EARLY_INIT
+#define MICROPY_BOARD_EARLY_INIT()
+#endif