This code is never compiled into U-Boot, so move it into a separate
file in tools/ to avoid the large #ifdef.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>

The FIT code is about half the size of the >3000-line image.c. Split this
code into its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>