Extended lfs_fs_gc to compact metadata, compact_thresh

This extends lfs_fs_gc to now handle three things:

1. Calls mkconsistent if not already consistent
2. Compacts metadata > compact_thresh
3. Populates the block allocator

Which should be all of the janitorial work that can be done without
additional on-disk data structures.

Normally, metadata compaction occurs when an mdir is full, and results in
mdirs that are at most block_size/2.

Now, if you call lfs_fs_gc, littlefs will eagerly compact any mdirs that
exceed the compact_thresh configuration option. Because the resulting
mdirs are at most block_size/2, it only makes sense for compact_thresh to
be >= block_size/2 and <= block_size.

Additionally, there are some special values:

- compact_thresh=0  => defaults to ~88% block_size, may change
- compact_thresh=-1 => disables metadata compaction during lfs_fs_gc

Note that compact_thresh only affects lfs_fs_gc. Normal compactions
still only occur when full.

tests/test_alloc.toml

@@ -7,6 +7,7 @@ if = 'BLOCK_CYCLES == -1'
 defines.FILES = 3
 defines.SIZE = '(((BLOCK_SIZE-8)*(BLOCK_COUNT-6)) / FILES)'
 defines.GC = [false, true]
+defines.COMPACT_THRESH = ['-1', '0', 'BLOCK_SIZE/2']
 code = '''
     const char *names[] = {"bacon", "eggs", "pancakes"};
     lfs_file_t files[FILES];
@@ -60,6 +61,7 @@ code = '''
 defines.FILES = 3
 defines.SIZE = '(((BLOCK_SIZE-8)*(BLOCK_COUNT-6)) / FILES)'
 defines.GC = [false, true]
+defines.COMPACT_THRESH = ['-1', '0', 'BLOCK_SIZE/2']
 code = '''
     const char *names[] = {"bacon", "eggs", "pancakes"};