linux.mk: .ub-File copying after building initramfs

test should exit with Exit-Code 0 if no .ub-File present and copy the
file if Exit-Code 1, otherwise make fails

Signed-off-by: Markus Kaindl <markus.kaindl@stusta.mhn.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>