loader: Support Flattened Image Trees (FIT images)

Introduce support for loading Flattened Image Trees, as used by modern
U-Boot. FIT images are essentially flattened device tree files which
contain binary images such as kernels, FDTs or ramdisks along with one
or more configuration nodes describing boot configurations.

The MIPS Boston board typically boots kernels in the form of FIT images,
and will make use of this code.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
[yongbok.kim@imgtec.com:
  fixed potential memory leaks,
  isolated building option]
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>