buildroot

linux: check that a repo and version is defined when using a custom repo

If not set the system will use an empty string which will result in
download errors for 'linux-.tar.gz' packages.

This patch makes it obvious to the user that the variable needs to be
set.

Signed-off-by: Christian Kellermann <christian.kellermann@solectrix.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>