wic: bootimg-efi: fix error handling

If we check for empty variables, it does not make sense to print their
content in case they are empty.

Additionally, the error message in the 'kernel' check attempted to print
the wrong variable ('target') which was not even defined, yet.

Also, raising WicError doesn't require an extra newline.

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>