tools/libfsimage: Cleanup makefiles

tools/libfsimage: Cleanup makefiles

Remove the need for "fs-*" targets by creating a "common.mk" which
have flags that are common to libfsimage/common/ and the other
libfsimages/*/ directories.

In common.mk, make $(PIC_OBJS) a recursively expanded variable so it
doesn't matter where $(LIB_SRCS-y) is defined, and remove the extra
$(PIC_OBJS) from libfsimage/common/Makefile.

Use a $(TARGETS) variable to list things to be built. And $(TARGETS)
can be use in the clean target in common.mk.

iso9660/:
    Remove the explicit dependency between fsys_iso9660.c and
    iso9660.h, this is handled automaticaly by the .*.d dependency files,
    and iso9660.h already exist.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>