In order to not execute code we just compiled, let's replace signrom
with a shell script that does the same thing while staying compatible
to pretty much every system available.

This should make cross-compilation for windows easier.

aliguori: fix build when objdir != srcdir
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>