ramips: do not stop the build if some targets fail

This becomes crucial when you are using a generic profile to build an image
larger than 4M, e.g. for w502u.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 35238