[package]: uboot-imx23: add new package for i.MX233 boards



git-svn-id: svn://svn.openwrt.org/openwrt@37822 3c298f89-4303-0410-b956-a3cf2f4a3e73
imx23: add support for u-boot image build



git-svn-id: svn://svn.openwrt.org/openwrt@37823 3c298f89-4303-0410-b956-a3cf2f4a3e73