buildroot

pkg-generic: check that SITE has a value when SOURCE

Currently, when an user forgets to define a SITE, he gets some fairly
weird download failure. In order to make things easier to diagnose, this
commit adds a check in the package infrastructure that verifies that if
SOURCE has a value, SITE is not empty.

Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>