This makes it easier to disable the shim build. (In Debian we need to
build the shim separately because it needs different compiler flags
and a different XEN_COMPILE_ARCH.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Gbp-Pq: Name 0039-tools-firmware-Makfile-Respect-caller-s-CONFIG_PV_SH.patch