This changes the ath10k firmware and driver selection, as
ath10k-ct is now the default in upstream OpenWrt.
However, for 802.11s operation we need the QCA firmware and driver.
ar71xx-generic and -tiny benefit most from the optimized kernel, as they
contain all devices with 32MB RAM. We enable CONFIG_GLUON_SPECIALIZE_KERNEL
for all targets using the mips24_kc architecture so packages shared between
targets don't need to be rebuild all the time.
The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.
Also fix targets without subtarget.
With a backported patch from the OpenWrt trunk, it is now easy to generate an
equivalent configuration using CONFIG_ALL_KMODS.
The build will take a bit longer because all kernel module packages are actually
built even when they are not included in the image, but adding new targets
becomes a lot easier.
Also, related documentation updates and fixes.
