procd: nand: make it possible to configure kernel and ubi partition

With this patch, the nand.sh script no longer overwrites
CI_KERNELPART and CI_UBIPART if they have been set before.

This allows platforms specify alternative values for these
variables.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

package/system/procd/files/nand.sh

@@ -5,10 +5,10 @@
 . /lib/functions.sh
 
 # 'kernel' partition on NAND contains the kernel
-CI_KERNPART="kernel"
+CI_KERNPART="${CI_KERNPART:-kernel}"
 
 # 'ubi' partition on NAND contains UBI
-CI_UBIPART="ubi"
+CI_UBIPART="${CI_UBIPART:-ubi}"
 
 ubi_mknod() {
 	local dir="$1"