[Matthias Schiffer: minor script improvements]

This new build flag is mandatory for now (it may default to 0 in a future
Gluon version). It may be set to the following values:

* 0 - Do not build any images for deprecated devices.
* upgrade - Only build sysupgrade images for deprecated devices.
* full - Build both sysupgrade and factory images for deprecated devices.

"Other" images are handled like factory images, as they are also used for
the initial installation of Gluon on a device.

The old bash-based parsing code was way too complex. Replace it with Lua.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

By passing the package name through merge_packages, it becomes possible to
override the package choice in GLUON_SITE_PACKAGES again, for example:

	GLUON_SITE_PACKAGES += -hostapd-mini hostapd

The domain configs are not checked yet, and not used for anything.

Based-on-patch-by: lemoer <git@irrelefant.net>

defaults to 11s, allowing to build devices without IBSS-support

obsoletes GLUON_ATH10K_MESH

Allow using relative paths for GLUON_SITEDIR, GLUON_OUTPUTDIR, ...

We also check for whitespace in paths now, as build will not work properly
with whitespace anyways, and Make's abspath would require escaping
otherwise.

[Matthias Schiffer: minor changes, rewrite commit message]

As the default package list is now handled using the 'packages' directive,
explicit wpad-mini removals in target definitions can be dropped.

This, together with a new try_config directive, will allow checking the
generic config and make it more flexible.

.SHELLSTATUS was not available before Make 4.2.

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.

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

`make list-targets` can be used to discover the available targets in
scripts.

OpenWrt does this by default to save some space.

Don't use system /tmp for build.

Leads to broken symlink setup by OpenWrt, usually breaking the build with
a "Too many levels of symbolic links" message.

Fixes #716.

Fixes some messages if /bin/sh doesn't support 'echo -e'.

Suggested-by: Steffen Möller <steffen_moeller@gmx.de>

This will allow us to use its content from other languages than Lua as
well.

Fixes #579

usign has pulled in new dependencies, and OpenWrt doesn't like the command
`make tools/prepare` in a clean build.

This will fix the early preparation happening much too often and also fixes
early preparation after `make clean`.