ath79: add support for YunCore TFTP image generation

ath79: add support for YunCore TFTP image generation

YunCore QCA9k based devices released in 2019 require a custom TFTP image
for U-Boot built-in recovery mode (triggered with reset button). Image
has to be prepended with 'YUNCORE' keyword followed by U-Boot CLI
commands which will be executed later. Images without the custom header
will be ignored by U-Boot.

To be able to support both the vendor firmware (QSDK) and OpenWrt flash
layouts, used here commands change the 'bootcmd' before flashing image.

This commit adds generic helper script for YunCore devices with 16 MB of
flash and enables TFTP image generation for A770 model.

Signed-off-by: Vincent Wiemann <vincent.wiemann@ironai.com>
[pepe2k@gmail.com: commit description reworded, recipe renamed]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
(cherry picked from commit 8016f648640e8f73f0dc4fc307fb6bebd12afb88)