ath79: consolidate Netgear EX7300 series images

ath79: consolidate Netgear EX7300 series images

This change consolidates Netgear EX7300 series devices into two images
corresponding to devices that share the same manufacturer firmware
image. Similar to the manufacturer firmware, the actual device model is
detected at runtime. The logic is taken from the netgear GPL dumps in a
file called generate_board_conf.sh.

Hardware details for EX7300 v2 variants
---------------------------------------
SoC: QCN5502
Flash: 16 MiB
RAM: 128 MiB
Ethernet: 1 gigabit port
Wireless 2.4GHz (currently unsupported due to lack of ath9k support):
- EX6250 / EX6400 v2 / EX6410 / EX6420: QCN5502 3x3
- EX7300 v2 / EX7320: QCN5502 4x4
Wireless 5GHz:
- EX6250: QCA9986 3x3 (detected by ath10k as QCA9984 3x3)
- EX6400 v2 / EX6410 / EX6420 / EX7300 v2 / EX7320: QCA9984 4x4

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>