source-check: change behavior to check only selected packages / add source-check-all

The original 'source-check' target first selects all packages, and then checks
whether the package tarballs can be found. This is useful for Buildroot
maintainers, but less useful for developers working on a specific project. The
latter only care about the packages used in that project.

This patch removes the allyesconfig dependency to source-check so that only
selected packages are checked. The original behavior is moved to a new target
'source-check-all'.

[Peter: removed source-check-all, people can just make allyesconfig before]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>