efi_loader: do not use 2.0.5 as UEFI revision number

Currently the UEFI revision number in the system table header is set to
2.0.5. This version number does not refer to any existing version of the
UEFI standard.

Set the revision number to 2.7.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Alexander Graf <agraf@suse.de>