Upstream removed the factory images for the Archer C20 v1 as they
potentially brick the device. Remove them from Gluon to avoid build
failures on the next OpenWrt 19.07 bump.

Signed-off-by: David Bauer <mail@david-bauer.net>

Use a manifest alias for the autoupdater image string of the Netgear
EX3700 / EX3800. This way, no unnecessary symlink is created and the
autoupdater functionality is preserved.

Specifications:
* SoC: MT7620A
* RAM: 64 MB DDR
* Flash: 8MB NOR SPI flash
* WiFi: MT7612E (5Ghz) and builtin MT7620A (2.4GHz)
* LAN: 1x100M

The -factory images can be flashed from the device's web
interface or via nmrpflash.

Both devices differ by a additional power outlet for the EX3800.
This patch has been tested on a EX3800 device.

Specification:
- MediaTek MT7620A (580 Mhz)
- 64 MB of RAM
- 8 MB of FLASH
- 2T2R 2.4 GHz and 1T1R 5 GHz
- 5x 10/100 Mbps Ethernet
- 2x external, non-detachable antennas
- UART (J1) header on PCB (115200 8n1)
- 8x LED (GPIO-controlled*), 2x button, power input switch
- 1 x USB 2.0 port

WiFi driver is stable now.

The old bash-based parsing code was way too complex. Replace it with Lua.

For some reason, some of the target files were executable.

As the default package list is now handled using the 'packages' directive,
explicit wpad-mini removals in target definitions can be dropped.