ramips: improve compatibility for Youku YK-L2 and YK-L1 series

ramips: improve compatibility for Youku YK-L2 and YK-L1 series

Add UIMAGE_NAME and UIMAGE_MAGIC to allow users to directly install
initramfs-kernel.bin from the stock firmware Web UI. At the same time,
this change makes it possible to boot OpenWrt with the official u-boot.

Notice:
Since the stock firmware is based on OpenWrt and the configuration
will be retained by default during the upgrade process, so we must use
initramfs-kernel.bin to do a initial installation. After the system
restarts, install sysupgrade.bin and do not retain any configuration.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>