Makefile: unbreak source-check

'source-check' breaks because the *_DEFCONFIG options are empty strings.
There is an explicit check to avoid erroring-out when doing 'source', but
not when doing 'source-check'.

This patch set DL_MODE right from the first sub-make call.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>