SW: enable grub-efi-amd64-bin/grub-efi-ia32-bin in GRMLBASE class [Closes: issue1124]

To get UEFI capabable ISOs we need grub-efi-amd64-bin/grub-efi-ia32-bin
in the build chroot. The packages aren't available on Debian/squeeze
but on >=Debian/wheezy, since that's what we really care about
put those packages into GRMLBASE so everyone gets it by default.