uboot-imx

buildman: adjust for Kconfig

Use "make <board>_defconfig" instead of "make <board>_config".

Invoke tools/genboardscfg.py to generate boards.cfg when it is missing.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>