Kconfig: Create a new tools menu

At present MKIMAGE_DTC_PATH is in the devicetree menu but not within
'devicetree control' since it does not relate to that. As a result it
shows up in the top menu.

It actually relates to the mkimage tool, so create a new tools menu for it
and move it there.

Signed-off-by: Simon Glass <sjg@chromium.org>