ramips: mt7621: tidy up names for Ubiquiti devices

ramips: mt7621: tidy up names for Ubiquiti devices

The "proper" vendor prefix for Ubiquiti is "ubnt", this is used in
all targets except ramips and also recommended by the kernel.

This patch adjusts the various board/image/device name variables
accordingly. Since we touch it anyway, this also adds the space
in "EdgeRouter X" as a hyphen to those variables to really make
them consistent with the model name.

While at it, create a real shared definition for the devices in
image/mt7621.mk instead of deriving one device from another.

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