Fix remote tracking branch for Buildroot submodule (home-assistant#2288)

Set 2022.02-haos as the default remote tracking branch. This should not
influence regular submodule updates/inits as they reference the git
hash tracked by the operating-system repository directly.

Author: agners

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
 	url = https://github.com/home-assistant/buildroot.git
-	branch = 2021.02.x-haos
+	branch = 2022.02.x-haos