binman: Add a separate section about environment variables

These are documented in various several sections. Add a new section that
mentions them all in one place so it is easier to see what environment
variables can be used to control U-Boot's use of binman.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>