ar71xx: add support for TP-Link RE450

Add full support for TP-Link RE450.
The wireless SoC is similiar to Archer C7: QCA9558 + QCA9880 (pci).
The ethernet interface is AR8035, but the mdio is connected to the gpio and
the chipset builtin mdio bus isn't used, which is unique (and weird), so the
kernel gpio mdio module is used.
The two ethernet leds are connected to the GPIO, so they are both configured,
one to indicate link status and the other to indicate data transfer.

The generation of the image was added to the image Makefile.
The return value of tplink-safeloader is not ignored anymore (to fail on error)

The result factory image is flashable from the stock web ui.

Signed-off-by: Tal Keren <kooolk@gmail.com>
[rd@radekdostal.com: ar71xx: RE450: do not build RE450 for nand]
Signed-off-by: Radek Dostál <rd@radekdostal.com>
[hyniu@o2.pl: ar71xx: RE450: Renaming LED accordance with the standard]
Signed-off-by: Henryk Heisig <hyniu@o2.pl>
[jo@mein.io: squash fixes from Radek and Henryk into original commit]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>