MAKEALL: adjust for Kconfig

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

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

 - Show "Building ${BOARD_NAME} board..." message.
   (Prior to Kconfig, instead, mkconfig script displayed
   "Configuring for ${BOARD_NAME} board..." but it was removed.)
   Without this message, we cannot know which board is currently
   being built.

 - Do not show "# configuration written to .config".
   This message is useless and just annoying for MAKEALL.

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