mkimage: unbreak build on non-linux systems

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35909 3c298f89-4303-0410-b956-a3cf2f4a3e73
kernel: add a missing config symbol for 3.8

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35910 3c298f89-4303-0410-b956-a3cf2f4a3e73