[new uImage] Pull in libfdt if CONFIG_FIT is enabled

New uImage format (Flattened Image Tree) requires libfdt
functionality, print out error message if CONFIG_OF_LIBFDT
is not defined.

New uImage support is enabled by defining CONFIG_FIT (and CONFIG_OF_LIBFDT).
This commit turns it on by default.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>