package: add toolchain dependency to every target package

This commit makes the dependency from the target toolchain explicit.
This way we can buid from command line a package that use
inner-generic-package right after the configuration phase, example:

	make clean <package-name>

Also remove TARGETS_ALL because the only purpose was to add toolchain
dependency so it's superseded by this commit.

To prevent circular dependency add the new variable
<pkgname>_ADD_TOOLCHAIN_DEPENDENCY to avoid adding the toolchain
dependency for toolchain packages.

This is also a step forward supporting top-level parallel make.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>