merge target kernel config files with subtarget config files and add a variable overr...

merge target kernel config files with subtarget config files and add a variable override for editing the target kernel config file with make kernel_menuconfig in case the subtarget contains overrides (thx, sn9)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@18362 3c298f89-4303-0410-b956-a3cf2f4a3e73