Previously `tests/pkg_littlefs` crashed on the `samr21-xpro`. This
now aligns the buffers in `littlefs_desc_t` to the alignment
requirement of `uint32_t`.
Specifically the issue causing the crash at hand was that
`lfs_free_t::buffer` is of type `uint32_t *`, so access are expected
to be aligned to `uint32_t`. After this commit, this assumption is
fulfilled.