core/pkg-infra: download git submodules if the package wants them

core/pkg-infra: download git submodules if the package wants them

Add a new package variable that packages can set to specify that they
need git submodules.

Only accept this option if the download method is git, as we can not get
submodules via an http download (via wget).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Aleksandar Simeonov <aleksandar@barix.com>
Tested-by: Matt Weber <matt@thewebers.ws>
Reviewed-by: Matt Weber <matt@thewebers.ws>
Tested-By: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>