rockchip: allow for multiple boot-scripts

Add support for select a bootscript depending on the device built. This
is necessary, as the FriendlyARM NanoPi R2S needs a different bootcmd in
order to produce output on the debug UART.

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/rockchip/image/Makefile

@@ -24,7 +24,7 @@ endef
 
 define Build/boot-script
 	# Make an U-boot image and copy it to the boot partition
-	mkimage -A arm -O linux -T script -C none -a 0 -e 0 -d mmc.bootscript $@.boot/boot.scr
+	mkimage -A arm -O linux -T script -C none -a 0 -e 0 -d $(if $(1),$(1),mmc).bootscript $@.boot/boot.scr
 endef
 
 define Build/pine64-img