Trigger generic board error only when building

At present the generic board error can occur when configuring U-Boot, or
during distclean, but this is incorrect. The existing autoconf.mk may come
from an earlier U-Boot configuration which is about to be overwritten.

Make the error conditional so that it will only be triggered when we are
actually building U-Boot.

This avoids a problem where the system is being reconfigured to remove
CONFIG_SYS_GENERIC_BOARD on an architecture that does not support it.
Currently this will print an error and require the manual removal of
include/autoconf.mk.

Signed-off-by: Simon Glass <sjg@chromium.org>