boot: Try to load UKI from simple filesystem before LoadImage

boot: Try to load UKI from simple filesystem before LoadImage

When the source buffer is NULL, the firmware is supposed to try to load the UKI
with simple filesystem protocol then load file 2 protocol. But it seems
on some versions of AMI, it does not use simple filesystem protocol,
and then fails to load if the ESP was loaded from an El Torito boot
catalog. Trying to load the source buffer from the simple filesystem protocol
protocols seems work around this limitation.

Shim for example, also loads the source buffer before calling LoadImage. So it
seems to be a safe thing to do. We could also maybe in the future use load file
2 protocol if simple filesystem failed in the first place.