'source' target should depend on 'dirs'

In the following command flow:
make distclean
make <board>_defconfig
make source

the dl directory would normally be created by the first wget download. If there are no such downloads, e.g. because you use a local download mirror (e.g. scp, file) or only use git/svn/hg/bzr repositories, the dl directory is not created automatically. This causes e.g. the 'pushd' command in the respective _DOWNLOAD commands to fail.

This patch adds a dependency to the 'dirs' target to 'source', fixing this scenario.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>