Allow Kernel/CompileImage/Initramfs to use a different rootfs location.
If the additional arg is not defined, TARGET_DIR is used by default.
This allows the caller to customize the kernel initramfs for different
rootfs.
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
[ simplify commit and rework commit description ]
Link: https://github.com/openwrt/openwrt/pull/12959
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>