tools: genboardscfg: move buildman path to first

To system which has kconfiglib installed, genboardscfg will
use system kconfiglib, we need it use U-Boot owned version,
so move the buildman path to first.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>