To reduce the number of packages that need to be listed in
GLUON_SITE_PACKAGES, this adds a new variable GLUON_FEATURES. Sets of
packages are enabled automatically based on the combination of listed
feature flags.

Site-specified package feeds can provide their own feature flag
definitions.

The Airgateway LR is the same device as the normal Airgateway but with an detachable antenna. The Pro version is a cheap dual band device.

These devices don't have unique MAC addresses with OpenWrt/LEDE at the moment
(see #993).

Fixes #961

Tested-by: Jannis Pinter <jannis@pinterjann.is>

The MR1750 and OM5P-AC devices are based on ath9k SoCs and an external
ath10k chip. All devices which are using ath10k should be marked as broken
due to deficits in their IBSS support.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

Fixes #842

+ added profile for these devices
+ added entry in the docs for these devices plus the missing alfa tube

The new MR1750v2 device support is only available in LEDE master. The
relevant patches have to backported to add support for them in Gluon

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

The new OM2P-HSv3 device support is only available in LEDE master. The
relevant patches have to backported to add support for them in Gluon

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>