'powerpc' is a valid arch for the LINUX_KARCH variable, but the build
system unconditionally replaces LIUNUX_KARCH=powerpc with
LINUX_KARCH=ppc.
This change only does the replacement if LINUX_KARCH isn't set. This
allows us to use the powerpc architecture.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@9405 3c298f89-4303-0410-b956-a3cf2f4a3e73