maint: ensure src/ directory includes are clean

maint: ensure src/ directory includes are clean

In 'make syntax-check', we have a rule that prevents layering
violations between the various files in src.  However, we
forgot to treat conf/ and the more recently-added access/ as
lower-level directories, and were not detecting cases where
they might have used a driver file.  Also, it's not nice that
qemu can use storage/ but none of the other drivers could do so.

* cfg.mk (sc_prohibit_cross_inclusion): Tighten rules for conf/
and access/, let all other drivers use storage/.

Signed-off-by: Eric Blake <eblake@redhat.com>