kirkwood: image: fix unwanted 2nd inclusion of kernel

kirkwood: image: fix unwanted 2nd inclusion of kernel

In commit d2e18dae2892 ("kirkwood: cleanup image build code") the image
build code was refactored, setting KERNEL_IN_UBI=0 which doesn't work as
the KERNEL_IN_UBI needs to be unset in order to make it working as
intended, which leads to factory images with two kernels in them:

 binwalk --keep-going openwrt-kirkwood-cisco_on100-squashfs-factory.bin
 MD5 Checksum:  c33e3d1eb0cb632bf0a4dc287592eb70
 DECIMAL   	HEX       	DESCRIPTION
 -------------------------------------------------------------------------------
 0         	0x0       	uImage header [...] "ARM OpenWrt Linux-4.14.123"
 5769216   	0x580800  	uImage header [...] "ARM OpenWrt Linux-4.14.123"

Cc: Mathias Kresin <dev@kresin.me>
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2285
Fixes: d2e18dae2892 ("kirkwood: cleanup image build code")
Signed-off-by: Petr Štetiar <ynezz@true.cz>