target/sdk: generate a Config.in file with the settings of the build that the SDK was generated from

This allows make oldconfig/menuconfig to run

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39864 3c298f89-4303-0410-b956-a3cf2f4a3e73