Fixes #1491

Addition of the OCEDO Koala to the supported devices in the docs was
forgotten in 52bc028e.

This device is a dual 5GHz device. It is recommended to manually change the
radio of the first device to the lower 5GHz channels and the second radio
to the upper 5GHz channels.

This commit adds support for the new ipq40xx target.

Fixes: #1463

add a missing command to set the special Gluon MAC address of the private WLAN interface to the documentation.

With all the packages being built 10GB are not enough anymore.

Closes: #1436

Let sites leave these texts empty by default, just add a comment where to
find the default labels.

Explain the magic of gluon.mk. The feature flag documention is moved into
this new page.