moveconfig: Allow querying board configuration

It is useful to be able to find out which boards define a particular
option, or combination of options. This is not as easy as grepping the
defconfig files since many options are implied by others.

Add a -f option to the moveconfig tool to permit this. Update the
documentation to cover this, including a better title for the doc page.

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