efivars: remove direct access to unaligned structure members

efivars: remove direct access to unaligned structure members

Most of the accesses *were* aligned. The only one that definetely wasn't was to
drive_path->part_start and drive_path->part_size, because those both expect
8 byte alignment, and are at offsets 4 and 12 in the packed structure.

Because of the way that device_path structure is defined and used, we expect
that device_path.length is always two-byte aligned.

This adds asserts in various places to ensure the proper alignment, and uses
memcpy in other places where the alignment might be off.