Bump Buildroot to 2021.02-rc3 (#1260)

* Rebase patches to Buildroot 2021.02-rc3

* Update Buildroot to 2021.02-rc3

* Declare Kernel headers to be Linux version 5.10 (since they are, and new Buildroot knows about 5.10)

buildroot/package/libglib2/0003-remove-cpp-requirement.patch

@@ -35,7 +35,7 @@ index 4bbf4c2..ac59f4e 100644
 @@ -1,4 +1,4 @@
 -project('glib', 'c', 'cpp',
 +project('glib', 'c',
-   version : '2.64.4',
+   version : '2.66.7',
    # NOTE: We keep this pinned at 0.49 because that's what Debian 10 ships
    meson_version : '>= 0.49.2',
 @@ -10,7 +10,6 @@ project('glib', 'c', 'cpp',