generic-board: make compile-time noise for non-generic boards

Commit 0f605c15 (Start the deprecation process for generic board)
added a run-time warning message.  Let's be noisier for non-generic
boards to inform the dead line of the conversion.

This commit intentionally outputs a warning message to stdout.
We still have many unconverted boards.  If we print the message
to stderr, MAKEALL and buildman will report tons of broken boards
and really important information will be buried.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>