libfdt: Move the working_fdt pointer to cmd_fdt.c

The working_fdt pointer was declared in common/fdt_support.c but was
not used there.  Move it to common/cmd_fdt.c where it is used (it is
also used in lib_ppc/bootm.c).

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>