fix grub2 installation on Debian Wheezy
fix grub2 installation on Debian Wheezy

--target has been introduced in grub2. This patch ensure we use
the minimal version before adding the --target XXX flag.

Debian Wheezy and Precise share the same limitation regarding the
root device.

Change-Id: I5b3ad42b9fb536d13fb65d84339bc15bc23d99a3

zioproto/diskimage-builder,varunarya10/diskimage-builder,eantyshev/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,NaohiroTamura/diskimage-builder,NaohiroTamura/diskimage-builder,rdo-management/diskimage-builder,eantyshev/diskimage-builder,JioCloud/diskimage-builder,varunarya10/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,JioCloud/diskimage-builder,switch-ch/diskimage-builder,rdo-management/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder