Update buildroot to 2021.02.2 (#1370)

buildroot/docs/manual/customize-rootfs.txt

@@ -60,6 +60,9 @@ The post-build scripts are run with the main Buildroot tree as current
 In addition, you may also use these environment variables:
 
   - +BR2_CONFIG+: the path to the Buildroot .config file
+  - +CONFIG_DIR+: the directory containing the .config file, and
+    therefore the top-level Buildroot Makefile to use (which is
+    correct for both in-tree and out-of-tree builds)
   - +HOST_DIR+, +STAGING_DIR+, +TARGET_DIR+: see
     xref:generic-package-reference[]
   - +BUILD_DIR+: the directory where packages are extracted and built