Makefile: binman: Search board directory before srctree

Makefile: binman: Search board directory before srctree

A file like rm-cfg.yaml accidentally left in the source tree root
shadows the board-specific copy. binman builds the wrong YAML, the
resulting rm-cfg.bin may match a different SoC, and we end up with
the following error:

  k3_system_controller sysctrler: k3_sysctrler_start:
      Boot Notification response failed. ret = -110

Move the board directory ahead of the srctree root so that the
most-specific match wins.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Simon Glass <sjg@chromium.org>