ipq806x: convert to dt-based board-detection

ipq806x: convert to dt-based board-detection

Use the generic board detection method:
- Board name: First compatible string from the device tree
- Board model: Model property from the device tree

Change occurrences of board name in userspace by the compatible
string, and removed target specific board detection script

Replace the definition of SUPPORTED_DEVICES in Device/Default
to extract the dt compatible string from each device definition.
Additionally, for devices supported by lede-17.01, append
the value of BOARD_NAME to SUPPORTED_DEVICES in the device
definition.

Signed-off-by: Luis Araneda <luaraneda@gmail.com>