stm32/main: Introduce MICROPY_HW_FLASH_MOUNT_AT_BOOT config option.

It's enabled by default to retain the existing behaviour.  A board can
disable this option if it manages mounting the filesystem itself, for
example in frozen code.
Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/main.c

@@ -153,7 +153,7 @@ STATIC mp_obj_t pyb_main(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_a
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(pyb_main_obj, 1, pyb_main);
 
-#if MICROPY_HW_ENABLE_STORAGE
+#if MICROPY_HW_FLASH_MOUNT_AT_BOOT
 // avoid inlining to avoid stack usage within main()
 MP_NOINLINE STATIC bool init_flash_fs(uint reset_mode) {
     if (reset_mode == 3) {
@@ -530,7 +530,7 @@ soft_reset:
     // Initialise the local flash filesystem.
     // Create it if needed, mount in on /flash, and set it as current dir.
     bool mounted_flash = false;
-    #if MICROPY_HW_ENABLE_STORAGE
+    #if MICROPY_HW_FLASH_MOUNT_AT_BOOT
     mounted_flash = init_flash_fs(reset_mode);
     #endif
 

ports/stm32/mpconfigboard_common.h

@@ -122,6 +122,11 @@
 #define MICROPY_HW_HAS_LCD (0)
 #endif
 
+// Whether to automatically mount (and boot from) the flash filesystem
+#ifndef MICROPY_HW_FLASH_MOUNT_AT_BOOT
+#define MICROPY_HW_FLASH_MOUNT_AT_BOOT (MICROPY_HW_ENABLE_STORAGE)
+#endif
+
 // The volume label used when creating the flash filesystem
 #ifndef MICROPY_HW_FLASH_FS_LABEL
 #define MICROPY_HW_FLASH_FS_LABEL "pybflash"