This changes the verification code in bootable/recovery to use
BoringSSL instead of mincrypt.
Cherry-pick of 452df6d99c, with
merge conflict resolution, extra logging in verifier.cpp, and
an increase in the hash chunk size from 4KiB to 1MiB.
Bug: http://b/28135231
Change-Id: I1ed7efd52223dd6f6a4629cad187cbc383d5aa84