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This changes the verification code in bootable/recovery to use
BoringSSL instead of mincrypt.

Cherry-pick of 452df6d9, 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