Improve error handling a bit, and fix upgrade for some (unsupported or
broken) targets.

.SHELLSTATUS was not available before Make 4.2.

gluon-mesh-babel is not merged yet, and once it is, this would lead to a
circular dependency.

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.

These packages don't work or don't make sense without batman-adv.

Update "scripts: metadata: always resolve dependencies through provides
list" to new version.

As PROVIDES can be used to replace real packages now, we don't need the
virtual packages as workaround anymore. This also means that the providing
packages don't need to be added to site.mk explicitly anymore when the
default provider is used.

Allow to use PROVIDES to replace existing packages.

LEDE has sha256sum enabled by default.

This reverts commit dcff6802.

The added debug information is stripped of during packaging, so it does not
increase the package size. It does however slightly change offsets in the
code, preventing the weird ash segfaults we've been seeing. Also, if the
issue returns, we'll have debug information matching the coredumps.

Fixes #1164

Based-on-patch-by: Christof Schulze <christof.schulze@gmx.net>

While we use the hexadecimal representation as a hash input for simplicity,
it should not be interpreted as case-sensitive.

The new proto will simplify the switch to VXLAN encapsulation.

The next-node MAC address doesn't need to be unique in different
communities, so we can as well add a default value.

`make show-release` can be used to discover the current release in scripts.

A bug in batman-adv can lead to a large amount of management traffic being
exchanged between nodes when the multicast optimizations are enabled,
effectively making the mesh unusable. It's safer to disable the feature
for now, until we have a real fix.