uboot-imx

binman: Add better Makefile debugging

There is a debugging option in the Makefile to allow people to figure out
which u-boot.dtsi files are used in the build. But is it not easy to use
since it only shows files it finds, not those it is looking for. Update it
and update the mention of it to the docs.

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