ath79: split DEVICE_TITLE in multiple variables

This allows the definition of DEVICE_VENDOR, DEVICE_MODEL and
DEVICE_VARIANT. All three are merged together to look the same as the
current DEVICE_TITLE. Also, if DEVICE_TITLE is set it's directly used as
a *fallback* for devices which weren't upgraded yet.

This is based on the work of @sudhanshu16.

The motivation is to create JSON files based on provided metadata,
needing clear differentiation between vendor, model and revision.

Signed-off-by: Paul Spooren <mail@aparcar.org>