Turn off mkfs FAT32 on all SAMD21 builds; note in doc

Author: dhalbert

ports/atmel-samd/mpconfigport.h

@@ -60,7 +60,9 @@
     X(EISDIR) \
     X(EINVAL) \
 
-#define MICROPY_FATFS_EXFAT 0
+#define MICROPY_FATFS_EXFAT    (0)
+// FAT32 mkfs takes about 500 bytes.
+#define MICROPY_FF_MKFS_FAT32 (0)
 
 // Only support simpler HID descriptors on SAMD21.
 #define CIRCUITPY_USB_HID_MAX_REPORT_IDS_PER_DESCRIPTOR (1)

shared-bindings/storage/__init__.c

@@ -268,7 +268,13 @@ STATIC const mp_rom_map_elem_t storage_module_globals_table[] = {
 //|
 //|     @staticmethod
 //|     def mkfs(self) -> None:
-//|         """Format the block device, deleting any data that may have been there"""
+//|         """Format the block device, deleting any data that may have been there.
+//|
+//|         **Limitations**: On SAMD21 builds, `mkfs()` will raise ``OSError(22)`` when
+//|         attempting to format filesystems larger than 4GB. The extra code to format larger
+//|         filesystems will not fit on these builds. You can still access
+//|         larger filesystems, but you will need to format the filesystem on another device.
+//|         """
 //|         ...
 //|     def open(self, path: str, mode: str) -> None:
 //|         """Like builtin ``open()``"""