buildscript: print warning instead of failing build

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>
Reviewed-by: Fabian Bläse <fabian@blaese.de>

buildscript

@@ -381,8 +381,8 @@ buildall() {
 }
 
 if [ "$(/usr/bin/id -u)" -eq 0 ]; then
-    echo "don't run buildscript as root"
-    exit 1
+    echo "WARNING: buildscript is running as root"
+    echo "WARNING: build may fail at a later stage"
 fi
 
 if [ "$1" != "selectbsp" -a "$1" != "selectvariant" ]; then