buildroot

Makefile: add a few more targets to nobuild_targets

With this commit, one can now execute the source-check, external-deps
and legal-info targets regardless of the checks normally being done by
packages on the configuration.

Note that we intentionally do not go down the road of adding %-source,
%-legal-info, and the miryad of other targets that could work in such
situations. We only whitelist a few targets that are really useful to
have as nobuild_targets.

[Thomas: also add 'clean' and 'distclean' to the nobuild_targets, as
suggested by Yann.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>