pkg/littlefs2: fix unaligned memory access
pkg/littlefs2: fix unaligned memory access

Previously `tests/pkg_littlefs2` crashed on the `samr21-xpro`. This
now aligns the buffers in `littlefs2_desc_t` to the alignment
requirement of `uint32_t`.

Specifically the issue causing the crash at hand was that
`struct lfs_free::buffer` is of type `uint32_t *`, so access are
expected to be aligned to `uint32_t`. After this commit, this
assumption is fulfilled.

miri64/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT