core/pkg-infra: check for an SCM in the correct variable

core/pkg-infra: check for an SCM in the correct variable

The site method is stored in FOO_SITE_METHOD, not in FOO_SITE.

Fixes:
http://autobuild.buildroot.net/results/131/13196dd779bc9e3b172c74851546dd4c4752aa02/

[Peter: add autobuilder reference]
Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>