image_signing: clean up UEFI signing/verifying code

Follow up the code review comments on CL:995175, which was merged as
1493e938e45535f86b7132a83123c6319eacb217
("image_signing: sign UEFI binaries")

BUG=b:62189155
TEST=See CL:*613656
BRANCH=none

Change-Id: Ic01bfbbfe39fbfb85c0f313ab62bbcd3e2fbb9a3
Reviewed-on: https://chromium-review.googlesource.com/1024919
Commit-Ready: Edward Jee <edjee@google.com>
Tested-by: Edward Jee <edjee@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
5 files changed