vboot: add documentation for different switches in vb2_api.h
vboot: add documentation for different switches in vb2_api.h

List two switches with their documentation in vb2_api.h:
- NEED_VB2_SHA_LIBRARY
- NEED_VB20_INTERNALS

BUG=b:124141368, b:124192753
TEST=Build locally
TEST=/work/vboot/src/repohooks/pre-upload.py
TEST=make clean && make runtests
TEST=make clean && COV=1 make coverage && make coverage_html
BRANCH=none

Change-Id: I91798f9211ddb09fa57cc958f6c9a662b9d41192
Signed-off-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1480751
Commit-Ready: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>
Tested-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@chromium.org>
Reviewed-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot