roms/Makefile: replace the $(EDK2_EFIROM) target with "edk2-basetools"

roms/Makefile: replace the $(EDK2_EFIROM) target with "edk2-basetools"

We don't (can't) have a recipe for building just $(EDK2_EFIROM);
therefore, while we call the target $(EDK2_EFIROM), we actually build all
of the edk2 BaseTools. Rename the target to edk2-basetools, and update the
iPXE prerequisite accordingly. This will let other targets depend on
"edk2-basetools", where an $(EDK2_EFIROM) pre-requisite would be
misleading.

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