* Considering that alerting users to potential security breaches that may be
exploited by boot media should also be performed by application that create
them, we add detection for all the currently known revoked UEFI bootloaders,
be it the ones from the official UEFI DBX as well as the ones from Windows'
SkuSiPolicy.p7b, and warn the user when one such bootloader is detected on
their source media.
* Note that, to actually be revoked, the bootloaders flagged through SkuSiPolicy
require the copying of the .p7b to the boot media, which we are currently
not enacting but will perform in a subsequent commit.
* Also fix a Coverity warning in hash.c.
