Rework SetInitramfs functions to take a second arg to define the
location of the .config. This is needed in preparation for PER_ROOTFS
Initramfs support as we will prepare .config in dedicated directory and
use them only later when the image is actually built.
Link: https://github.com/openwrt/openwrt/pull/12959
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
