makefs: tidy up reach-over source

• cd9660 relies on an #include "iso.h" but does not build any .c files out of source, so remove reach-over .PATH
• ffs does not rely on any sys/ headers, so remove -I from CFLAGS.
• ffs_tables from sys/ is used by ffs; move the SRCS entry from the top- level Makefile to ffs' Makefile.inc.

Sponsored by: The FreeBSD Foundation