Harden shim against non-participating bootloaders.

It works like this: during startup of shim, we hook into the system's
ExitBootServices() and StartImage().  If the system's StartImage() is
called, we automatically unhook, because we're chainloading to something
the system can verify.

When shim's verify is called, we record what kind of certificate the
image was verified against.  If the call /succeeds/, we remove our
hooks.

If ExitBootServices() is called, we check how the bootloader verified
whatever it is loading.  If it was verified by its hash, we unhook
everything and call the system's EBS().  If it was verified by
certificate, we check if it has called shim_verify().  If it has, we
unhook everything and call the system's EBS()

If the bootloader has not verified anything, and is itself verified by
a certificate, we display a security violation warning and halt the
machine.

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