efi_loader: support device tree for bootefi selftest

The second argument of the bootefi command should always be usable to
specify a device tree. This was missing for bootefi selftest and
bootefi hello.

Proper error handling is added.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>