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>