Create a new disk for use with tests, which contains the new 'testapp'
EFI app specifically intended for testing the EFI loader.
Attach it to the USB device, since most testing is currently done with
mmc.
Initially this image will be used to test the EFI bootmeth.
Fix a stale comment in prep_mmc_bootdev() while we are here.
For now this uses sudo and a compressed fallback file, like all the
other bootstd tests. Once this series is in, the patch which moves
this to use user-space tools will be cleaned up and re-submitted.
Signed-off-by: Simon Glass <sjg@chromium.org>