Add persistent VFS shares so that the devices can be used with USB

that is present across soft-reset.

atmel-samd/mpconfigport.h

@@ -70,6 +70,7 @@
 #define MICROPY_FATFS_RPATH            (2)
 #define MICROPY_FATFS_VOLUMES          (4)
 #define MICROPY_FATFS_MULTI_PARTITION  (1)
+#define MICROPY_FATFS_NUM_PERSISTENT   (1)
 #define MICROPY_FSUSERMOUNT            (1)
 // Only enable this if you really need it. It allocates a byte cache of this
 // size.

py/mpconfig.h

@@ -403,6 +403,11 @@
 #define MICROPY_READER_FATFS (0)
 #endif
 
+// Number of VFS mounts to persist across soft-reset.
+#ifndef MICROPY_FATFS_NUM_PERSISTENT
+#define MICROPY_FATFS_NUM_PERSISTENT (0)
+#endif
+
 // Hook for the VM at the start of the opcode loop (can contain variable
 // definitions usable by the other hook functions)
 #ifndef MICROPY_VM_HOOK_INIT

py/runtime.c

@@ -103,7 +103,8 @@ void mp_init(void) {
 
     #if MICROPY_FSUSERMOUNT
     // zero out the pointers to the user-mounted devices
-    memset(MP_STATE_VM(fs_user_mount), 0, sizeof(MP_STATE_VM(fs_user_mount)));
+    memset(MP_STATE_VM(fs_user_mount) + MICROPY_FATFS_NUM_PERSISTENT, 0,
+           sizeof(MP_STATE_VM(fs_user_mount)) - MICROPY_FATFS_NUM_PERSISTENT);
     #endif
 
     #if MICROPY_PY_THREAD_GIL