Fix FIT and FDT support to have CONFIG_OF_LIBFDT and CONFIG_FIT independent

FDT support is used for both FIT style images and for architectures
that can pass a fdt blob to an OS (ppc, m68k, sparc).

For other architectures and boards which do not pass a fdt blob to an
OS but want to use the new uImage format, we just need FIT support.

Now we can have the 4 following configurations :

1) FIT only             CONFIG_FIT
2) fdt blob only        CONFIG_OF_LIBFDT
3) both                 CONFIG_OF_LIBFDT & CONFIG_FIT
4) none                 none

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>