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`.

This broke due to the Makefile changes for building the module repo.

`make modules` will generate a opkg repository containing kernel modules
matching the kernel of the Gluon images.

As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

Add an optional third argument to the GluonProfile macro while contains the
OpenWrt profile name. This allows creating different Gluon profiles (with
different image names etc.) from the same OpenWrt profile.

The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.