ar71xx: move Arduino Yun to generic building code

ar71xx: move Arduino Yun to generic building code

Migrate Arduino Yun from legacy to generic building code.

Note: the mtd partitioning is changed to adopt the LEDE default
partitioning. It allows to have a kernel bigger than 1280k. It is
necessary as kernel > 4.4 with default LEDE configuration grows
bigger.

To use the new partitioning, you need to update your U-Boot env in
advance:

setenv mtdparts "spi0.0:256k(u-boot)ro,64k(u-boot-env),15936k(firmware),64k(nvram),64k(art)ro"
setenv bootcmd "run addboard; run addtty; run addparts; run addrootfs; bootm 0x9f050000 || bootm 0x9fea0000"
saveenv

Signed-off-by: Camille Bilodeau <camille.bilodeau@protonmail.com>