build: image: add SOC device variable

build: image: add SOC device variable

This creates the device variable SOC and adds it to DEFAULT_DEVICE_VARS.

It is supposed to replace target-specific SOC variables like ATH_SOC or
MTK_SOC and thus unify variable names across targets.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>