Makefile: Fix message for missing configure

Makefile: Fix message for missing configure

When make is called without a valid configuration,
it should tell the user what to do.

Revision 0e8c9214ba1d4128cf92442cd343bc3733478261
was a regression which resulted in a message
which was no longer user friendly
(reported by Aurelien Jarno).

This patch restores the old behaviour.

Cc: Aurelien Jarno <[email protected]>
Reviewed-by: Andreas Färber <[email protected]>
Signed-off-by: Stefan Weil <[email protected]>
Signed-off-by: Aurelien Jarno <[email protected]>