ath79: redistribute UBNT variables in Ubiquiti image Makefile

ath79: redistribute UBNT variables in Ubiquiti image Makefile

The variables UBNT_BOARD and UBNT_VERSION are defined in the parent
Device/ubnt definition and then overwritten for most of the derived
platform definitions (e.g. Device/ubnt-wa).

Since this mixed use of inheritance and overwriting can be misleading,
this moves the variables to the platform-based definitions.

While at it, reorder the definitions to have order consistent, too.

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