ar71xx: initial support for TP-Link Archer C7v2

The C7v2 has 16 MB flash and QCA9880-BR4A rev 2 supported by ath10k driver.
The C7v1 had 8 MB flash and the unsupported QCA9880-AR1A rev 1.

Signed-off-by: Adam Serbinski <adam@serbinski.com>
Patchwork: http://patchwork.openwrt.org/patch/5071/
[juhosg:
 - remove the v2 specific profile add the ath10k driver to the existing
   Archer C7 profile instead. Although on v1 devices it does not change
   the non-working behaviour, but the ath10k driver is useful for users
   whom have replaced the wifi card with a supported one in their units.
 - update image/Makefile to build firmware image for both boards if the
   Archer C7 profile is selected]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40463 3c298f89-4303-0410-b956-a3cf2f4a3e73
ar71xx: add comments for image Makefile.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/5075/
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40464 3c298f89-4303-0410-b956-a3cf2f4a3e73