[new uImage] Do not compile new uImage format support by default

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>