sha-1 has known collision issues, which may not be readily
exploitable yet (in our context), but we should ideally use
a more secure method for checking file integrity.
therefore, use sha-2 (sha512sum) for checking files. this is
slower than sha-1, but checksum verification is only a minor
part of what lbmk does, so the overall effect on build times
is quite negligible.
Signed-off-by: Leah Rowe <leah@libreboot.org>
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