ipq806x: improve model name and revision for TP-Link AD7200

ipq806x: improve model name and revision for TP-Link AD7200

The TP-Link AD7200 appears with and without the "Talon" model name
prefix. Let's use both variants for 'make menuconfig' so everybody
can locate the device.

Concerning the revision, the TP-Link page lists v1 and v2 with the
device currently marked as "End of Life". However, the v2 and latest
v1 firmware are byte-identical. Thus, we only need one image for
this device and do not need to include the revision in the image name.

While at it, remove the useless BOARD_NAME variable which only makes
sense in combination with upgrade from legacy stable versions or when
custom upgrade scripts are involved.

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