ramips: append tail to WF2881 initramfs image

ramips: append tail to WF2881 initramfs image

Stock firmware has a vendor-defined tail at the end of uImage for image
validation. This patch enables OpenWrt installation from stock firmware
without having to access the UART console.

Installation via web interface:
1.  Flash **initramfs** image through the stock web interface.
2.  Boot into OpenWrt and perform sysupgrade with sysupgrade image.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
(cherry picked from commit be3e98ce26da5e737744427e1d59dd1c0508edef)