debugging: do not require no stripping

The stripping options operate on the final image and not the intermediate
builds, so requiring stripping to be disabled just to enable debugging
options doesn't make much sense.  Especially when working with gdbserver:
only the host needs the debugging information to be available.  The board
can run & debug perfectly fine without it.

Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>