stand: geli CFLAGS tightening

Only add -DWEAK_REFS to sha256.c and sha512.c instead of
everything. Remove redundant include that's not needed.
Minor formatting tweak.

Sponsored by: Netflix

(cherry picked from commit eaf7aabddcde9bf9eecffb41ba6569de0f5fa645)

stand geli: Restore include path to LDRSRC.

Various GELI sources need bootstrap.h and disk.h. In theory they
shouldn't need anything outside of libsa, but disk.h and bootstrap.h are
currently required.

This fixes the build with MK_LOADER_ZFS=no.

Obtained from: CheriBSD
Fixes: eaf7aabddcde stand: geli CFLAGS tightening
Sponsored by: DARPA
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35861

(cherry picked from commit bb7ddd077c2528b37399304d7a9bf70b25c42b47)