binman: cst: Build from source

binman: cst: Build from source

Build the imx code singing tool from source instead of relying on the
distro to provide the tool.
Use the debian/unstable branch because the default branch is outdated.
The binary is supposed to be build with docker, work around that by selecting
the correct Makefile directly.
Also append the description and add a link to documentation.

Signed-off-by: Leonard Anderweit <l.anderweit@phytec.de>
Reviewed-by: Simon Glass <sjg@chromium.org>