setup-defconfig: fix bashism

Reported by Francesco Del Degan

Signed-off-by: Denys Dmytriyenko <denys@ti.com>

recipes-kernel/linux/setup-defconfig.inc

@@ -37,7 +37,7 @@ do_configure() {
                 exit 1
             else
                 # Sanitize config fragment files to be relative to sources
-                configfrags+=" ${S}/$f"
+                configfrags="$configfrags ${S}/$f"
             fi
         done
     fi