extmod/vfs_lfsx: Allow overriding the LFS2 on-disk version format.

Back in LFS2 version 2.6 they updated the on-disk version from 2.0 to 2.1
which broke back compatibility (aka older versions could no long read new
version disk format), see
https://github.com/littlefs-project/littlefs/releases/tag/v2.6.0

Then in LFS2 v2.7 an optional `config->disk_version` was added to force the
library to use an older disk format instead, see:
https://github.com/littlefs-project/littlefs/releases/tag/v2.7.0

This commit simply exposes `config->disk_version` as a compile time option
if LFS2_MULTIVERSION is set, otherwise there is no change in behavior.
This is Useful for compatibility with older LFS versions.

Note: LFS2_MULTIVERSION needs to be defined at the make / CFLAGS level,
setting it in mpconfigboard.h doesn't work as it's not included in the
`lfs2.c` file in any way.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

extmod/vfs_lfsx.c

@@ -104,6 +104,12 @@ static void MP_VFS_LFSx(init_config)(MP_OBJ_VFS_LFSx * self, mp_obj_t bdev, size
     config->read_buffer = m_new(uint8_t, config->cache_size);
     config->prog_buffer = m_new(uint8_t, config->cache_size);
     config->lookahead_buffer = m_new(uint8_t, config->lookahead_size);
+    #ifdef LFS2_MULTIVERSION
+    // This can be set to override the on-disk lfs version.
+    // eg. for compat with lfs2 < v2.6 add the following to make:
+    // CFLAGS += '-DLFS2_MULTIVERSION=0x00020000'
+    config->disk_version = LFS2_MULTIVERSION;
+    #endif
     #endif
 }