Kconfig: Create a menu for FIT

This is a major feature with a lot of options. Give it its own menu to
tidy up the 'make menuconfig' display. Drop the 'depends on FIT' pieces
which are now unnecessary, since they are now bracketed by an 'if FIT'.

Leave CONFIG_TIMESTAMP out since it affects legacy images too.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>