Unlike other subprojects, these require an overlay directory to include
meson rules to build the libraries.  The rules are basically lifted
from tests/fp/meson.build, with a few changes to create platform.h
and publish a dependency.

The build defines are passed through a subproject option, and posted
back to users of the library via the dependency's compile_args.

The only remaining user of GIT_SUBMODULES and GIT_SUBMODULES_ACTION
is roms/SLOF, which is used to build pc-bios/s390-ccw.  All other
roms submodules are only present to satisfy the license on pre-built
firmware blobs.

Best reviewed with --color-moved.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>