Read kernel branch parameter

config/sources/families/rockchip-rk3588.conf

@@ -24,7 +24,7 @@ case $BRANCH in
 		KERNELDIR='linux-rockchip64'
 		KERNELSOURCE='https://github.com/armbian/linux-rockchip'
 		declare -g KERNEL_MAJOR_MINOR="5.10" # Major and minor versions of this kernel.
-		KERNELBRANCH='branch:rk-5.10-rkr4'
+		[[ -z "${KERNELBRANCH}" ]] && KERNELBRANCH='branch:rk-5.10-rkr4'
 		KERNELPATCHDIR='rockchip-rk3588-legacy'
 		;;