Fixes #1022

Fixes a build issue when switching targets.

Setting DEVICES will also suppress cleaning the package directory,
improving incremental build speed.

A lot of our code does not actually work with other prefix lengths.

Fixes #1014

Add gluon-dns-config: a package that enables the node to be used as DNS cache

The ar71xx-tiny target sets CONFIG_CLEAN_IPKG, which would delete opkg
control files required for user creation. Fix image generation and the user
creation script.

Fixes #1012

Fix an exception breaking the page.

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

ath10k has to switch the regulatory domain when switching the default
country when ATH_USER_REGD is enabled. This is for example important when
switching from country US (FCC) to DE (ETSI). The ath implementation will
use ath_reg_dyn_country_user_allow to check if this is allowed.

Unfortunately, this function always seems to return false to
ath_reg_notifier_apply even when ATH_USER_REGD is enabled. But it must
actually always accept the requests from the user (when ATH_USER_REGD is
enabled) to correctly set the conformance test limits in the ath10k binary
blob.

Not doing it will sometimes allow too high transmit powers on edge channels
for correctly calibrated devices outside their default regulatory domain.

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

Closes #844

gluon-core: add function to gluon.util that returns all active mesh-devices

Fixes #961

gluon-core: add get_mesh_devices(uconn) to gluon.util which will return all devices being up and having proto = gluon_mesh

Backport a few patches to allow sysupgrades on mmcblk and similar block
devices.