Makefile: provide sah-key to binman

Set the variable pre-load-key-path with the shell variable
PRE_LOAD_KEY_PATH that contain the keys path (used for signature).
This variable pre-load-key-path is provided to binman.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>