powerpc/install: Bail with error code on error in install script

If anything goes wrong when copying images into the install path, then
the install script should exit with an error code so that 'make' knows
about it and tells the user.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>