Added out-of-order write testing to emubd

Some forms of storage, mainly anything with an FTL, eMMC, SD, etc, do
not guarantee a strict write order for writes to different blocks. It
would be good to test that this doesn't break littlefs.

This adds LFS_EMUBD_POWERLOSS_OOO to lfs_emubd, which tells lfs_emubd to
try to break any order-dependent code on powerloss.

The behavior right now is a bit simple, but does result in test
breakage:

1. Save the state of the block on first write (erase really) after
   sync/init.

2. On powerloss, revert the first write to its original state.

This might be a bit confusing when debugging, since the block will
appear to time-travel, but doing anything fancier would make emubd quite
a bit more complicated.

You could also get a bit fancier with which/how many blocks to revert,
but this should at least be sufficient to make sure bd sync calls are in
the right place.

tests/test_seek.toml

@@ -329,6 +329,10 @@ code = '''
 # must be power-of-2 for quadratic probing to be exhaustive
 defines.COUNT = [4, 64, 128]
 reentrant = true
+defines.POWERLOSS_BEHAVIOR = [
+    'LFS_EMUBD_POWERLOSS_NOOP',
+    'LFS_EMUBD_POWERLOSS_OOO',
+]
 code = '''
     lfs_t lfs;
     int err = lfs_mount(&lfs, cfg);