Disable default building of new uImage format support in preparation
for merge with the master. Support for new format can be enabled on
a per-board basis, by defining the following in the board's config file:

#define CONFIG_FIT             1
#define CONFIG_OF_LIBFDT       1

This can be optionally defined to give more verbose output:

#define CONFIG_FIT_VERBOSE     1 /* enable fit_format_{error,warning}() */

Signed-off-by: Bartlomiej Sieka <tur@semihalf.com>