roms: build edk2 firmware binaries and variable store templates

roms: build edk2 firmware binaries and variable store templates

Add the "efi" target to "Makefile".

Introduce "Makefile.edk2" for building and cleaning the firmware images
and varstore templates.

Collect the common bits from the recipes in the helper script
"edk2-build.sh".

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>