Fixed issue with aggressively rounding down lookahead configuration

The littlefs allows buffers to be passed statically in the case
that a system does not have a heap. Unfortunately, this means we
can't round up in the case of an unaligned lookahead buffer.

Double unfortunately, rounding down after clamping to the block device
size could result in a lookahead of zero for block devices < 32 blocks
large.

The assert in littlefs does catch this case, but rounding down prevents
support for < 32 block devices.

The solution is to simply require a 32-bit aligned buffer with an
assert. This avoids runtime problems while allowing a user to pass
in the correct buffer for < 32 block devices. Rounding up can be
handled at higher API levels.

lfs.h

@@ -236,7 +236,6 @@ typedef struct lfs_superblock {
 } lfs_superblock_t;
 
 typedef struct lfs_free {
-    lfs_size_t lookahead;
     lfs_block_t begin;
     lfs_block_t end;
     lfs_block_t off;