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>

config.mk

diff --git a/config.mk b/config.mk
index 89a89f2..b427a4e 100644
--- a/config.mk
+++ b/config.mk
@@ -233,8 +233,8 @@
 # Does this architecture support generic board init?
 ifeq ($(__HAVE_ARCH_GENERIC_BOARD),)
 ifneq ($(CONFIG_SYS_GENERIC_BOARD),)
-$(error Your architecture does not support generic board. Please undefined \
-CONFIG_SYS_GENERIC_BOARD in your board config file)
+CHECK_GENERIC_BOARD = $(error Your architecture does not support generic board. \
+Please undefined CONFIG_SYS_GENERIC_BOARD in your board config file)
 endif
 endif