Makefile: move source-check outside of noconfig_targets

make source-check is here to check whether the remote sources for the
current selection of packages are still available. In its current
implementation, since it simply calls recursively a sub-make with the
source target, it can be a noconfig_targets. However, a follow-up
change will make source-check not use a sub-make, which will require
it to no longer be a noconfig_targets.

Therefore, as a preparation, this commit moves source-check outside of
noconfig_targets.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>