Use GNU stat (as determined by OpenWRT) for GLUON_CONFIG_VERSION

This *should* fix the build on non-Linux systems like MacOS X.
make: nicer gluon-tools dependencies
make: remove unneeded rule dependency (it is already defined in openwrt/tools/Makefile)
Allow building single targets (like single packages etc.)
Return exit code on missing value

Let the site configuration value script return 1 when
the requested value is not available.This enables us
to react on failures, so we can for example use a default
value.

With this patch the script will return with exitcode 1 if
the value is not found and no output on stdout. Otherwise
exitcode is 0 and the value is returned on stdout.
Make the version repo site configurable

Introduce a new value "opkg_repo" in the site configuration.
The value should be a single domain name,
e.g. "http://opkg.services.ffki/attitude_adjustment/12.09/%S/packages".
make: include the SUBTARGET in BOARD_BUILDDIR

Also, more cleanup.
Some work towards multi-target support
More multi-target support (still not finished...)
Fix initial build
Another build fix

Explicitly re-build prepare-tmpinfo after the feeds have been installed, as it
isn't done automatically anymore since we have the gluon-tools target.
README: some info about updating and rebuilding Gluon
README: autoupdater
Include rules.mk earlier so STAGING_DIR_HOST is available in gluon.mk
Generate .config without symlink again

Otherwise, `make cleanall` fails...
Update gluon packages
Use target-specific openwrt directories with symlinks

This allows us to use separate .config files per target. Also, don't use
OpenWRT's feeds script anymore, but symlink the feeds ourselves.
Fix cleanall target
Remove prepared stamp on make clean
make: don't install with --force-*, we want to know when something breaks