efibuild: Allow for manual control of audk commit

[mikebeaton]

Hi @vit9696 -

I don't reallly like what I've done here, however it is very useful!

For instance, in investigating acidanthera/bugtracker#2507 (comment), I was able to run:

rm -rf UDK
AUDK=f39c4bc76a5bc4904159996dd752ac4bb4601a72 ./build_duet.tool 
AUDK=f39c4bc76a5bc4904159996dd752ac4bb4601a72 ./build_oc.tool

to rebuild using acidanthera/audk@f39c4bc, and then

rm -rf UDK
AUDK=21e77e TARGETS=RELEASE ARCHS=X64 ./build_duet.tool 
AUDK=21e77e TARGETS=RELEASE ARCHS=X64 ./build_oc.tool --skip-tests

for quick rebuild, RELEASE only, using acidanthera/audk@21e77e0.