env: Move env_get_yesno() to env.h

Move env_get_yesno() over to the new header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

cmd/source.c

@@ -16,6 +16,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <image.h>
 #include <malloc.h>
 #include <mapmem.h>