makefiles: Reject 2>&1 > /dev/null

While this could theoretically be desired, it's usually just a mishap.
It is unlikely that legitimate cases will be needed in the build system;
if so, they can exclude themselves.

See-Also: https://github.com/RIOT-OS/RIOT/pull/16775

dist/tools/buildsystem_sanity_check/check.sh

@@ -349,6 +349,19 @@ check_no_riot_config() {
         | error_with_message "Don't push RIOT_CONFIG_* definitions upstream. Rather define configuration via Kconfig"
 }
 
+check_stderr_null() {
+    local patterns=()
+    local pathspec=()
+
+    patterns+=(-e '2>[[:blank:]]*&1[[:blank:]]*>[[:blank:]]*/dev/null')
+
+    pathspec+=('Makefile*')
+    pathspec+=('**/Makefile*')
+    pathspec+=('**/*.mk')
+    git -C "${RIOTBASE}" grep -n "${patterns[@]}" -- "${pathspec[@]}" \
+        | error_with_message "Redirecting stderr and stdout to /dev/null is \`>/dev/null 2>&1\`; the other way round puts the old stderr to the new stdout."
+}
+
 error_on_input() {
     ! grep ''
 }
@@ -369,6 +382,7 @@ all_checks() {
     check_no_usemodules_in_makefile_include
     check_no_pkg_source_local
     check_no_riot_config
+    check_stderr_null
 }
 
 main() {