Extended lfsr_format with some gc flags

These fall out quite naturally when you consider that we call
lfsr_mountinited internally to check that our format was successful.

That being said... they don't really do anything right now since we only
write a single mdir:

- LFS_F_COMPACT - The only gc operation that _might_ actually do
  something is LFS_F_COMPACT, but only if our fs config exceeds >1/2 the
  block size. But I'm not sure littlefs will even be able to write file
  metadata if this happens...

- LFS_F_CKMETA - We already check the only mdir by calling
  lfsr_mountinited, which implicitly fetches the mrootanchor.

- LFS_F_CKDATA - We uh, don't have any data immediately after
  lfsr_format. But I guess it doesn't hurt to keep this around for
  consistency, it at least implies CKMETA.

Hopefully these flags will be more interesting if/when we start adding
auxiliary trees to the filesystem, otherwise they may be worth reverting
in the future...

Until then, they at least provide some consistency, and I guess a way to
triply check that format was successful.

---

This could probably be better deduplicated, but calling lfsr_fs_gc from
both lfsr_mount and lfsr_format provides a bit better code organization:

           code          stack
  before: 36448           2680
  after:  36480 (+0.1%)   2680 (+0.0%)

lfs.h

@@ -158,6 +158,11 @@ enum lfs_type {
 #define LFS_F_CKREADS   0x00000020  // Check reads via parity bits/checksums
 #endif
 
+#define LFS_F_MTREEONLY 0x00010000  // Only traverse the mtree
+#define LFS_F_COMPACT   0x00000800  // Compact metadata logs
+#define LFS_F_CKMETA    0x00001000  // Check metadata checksums
+#define LFS_F_CKDATA    0x00002000  // Check metadata + data checksums
+
 // Filesystem mount flags
 #define LFS_M_RDWR               0  // Mount the filesystem as read and write
 #define LFS_M_RDONLY             1  // Mount the filesystem as read only