esp32/main: Make the entry point function name configurable.

This commit introduces a new port configuration entry allowing the entry
point function name to be changed, from "app_main" to a custom name.

This is needed when MicroPython is embedded as an ESP-IDF component,
since the "app_main" symbol is already provided elsewhere, making
compilation not possible.  Marking MicroPython's symbol as weak would
make it compile and make it possible to create and start the MicroPython
task anyway with the right FreeRTOS task creation incantation, but it is
probably easier to just rename the initialisation function into
something else that can be accessed from outside.

When MicroPython is embedded as an ESP-IDF component, the
MICROPY_ESP_IDF_ENTRY definition can be set to indicate the new entry
point function name.  The new function name prototype should still be
defined in external code to let linking succeed.

Also, the NLR failure callback is marked as weak to give the chance of
handling such error in a more controlled fashion rather than trigger an
unconditional board restart.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

ports/esp32/main.c

@@ -216,7 +216,7 @@ void boardctrl_startup(void) {
     }
 }
 
-void app_main(void) {
+void MICROPY_ESP_IDF_ENTRY(void) {
     // Hook for a board to run code at start up.
     // This defaults to initialising NVS.
     MICROPY_BOARD_STARTUP();
@@ -225,7 +225,7 @@ void app_main(void) {
     xTaskCreatePinnedToCore(mp_task, "mp_task", MICROPY_TASK_STACK_SIZE / sizeof(StackType_t), NULL, MP_TASK_PRIORITY, &mp_main_task_handle, MP_TASK_COREID);
 }
 
-void nlr_jump_fail(void *val) {
+MP_WEAK void nlr_jump_fail(void *val) {
     printf("NLR jump failed, val=%p\n", val);
     esp_restart();
 }

ports/esp32/mpconfigport.h

@@ -391,3 +391,8 @@ void boardctrl_startup(void);
 #ifndef MICROPY_PY_STRING_TX_GIL_THRESHOLD
 #define MICROPY_PY_STRING_TX_GIL_THRESHOLD  (20)
 #endif
+
+// Code can override this to provide a custom ESP-IDF entry point.
+#ifndef MICROPY_ESP_IDF_ENTRY
+#define MICROPY_ESP_IDF_ENTRY app_main
+#endif