ramips/mt76x8: add initramfs support for tplink 841n v13

The tplink 841n v13 requires an tplink v2 image header in
front of an initramfs image.

To boot an initramfs image:
- break the uboot by holding the '4' key
- setup your tftp server 192.168.0.255
- tftp 81000000 lede-ramips-mt76x8-tl-wr841n-v13-initramfs-kernel.bin
- bootm

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>