dist/tools/buildsystem_sanity_check: add an export variable check

Check that some variables are not exported in the build system.
This should track variables that managed to not be exported anymore so
that they do not reappear in a BSP.

It is not a whitelist but just a way to keep things cleaned in the
future.

dist/tools/buildsystem_sanity_check/check.sh

@@ -41,11 +41,38 @@ check_not_parsing_features() {
     git -C "${RIOTBASE}" grep "${patterns[@]}" -- "${pathspec[@]}"
 }
 
+# Some variables do not need to be exported and even cause issues when being
+# exported because they are evaluated even when not needed.
+#
+# Currently this blacklists exported variables instead of whitelisting or
+# providing a mechanism for handling it.
+# It just keep things not exported anymore in the future.
+UNEXPORTED_VARIABLES=()
+UNEXPORTED_VARIABLES+=('FLASHFILE')
+UNEXPORTED_VARIABLES+=('TERMPROG' 'TERMFLAGS')
+check_not_exporting_variables() {
+    local patterns=()
+    local pathspec=()
+
+    for variable in "${UNEXPORTED_VARIABLES[@]}"; do
+        patterns+=(-e "export[[:blank:]]\+${variable}")
+    done
+
+    pathspec+=('*')
+
+    # Ignore `makefiles/vars.inc.mk` as it currently is the only place that
+    # should export common variables.
+    pathspec+=(':!makefiles/vars.inc.mk')
+
+    git -C "${RIOTBASE}" grep "${patterns[@]}" -- "${pathspec[@]}"
+}
+
 
 main() {
     local errors=''
 
     errors+="$(check_not_parsing_features)"
+    errors+="$(check_not_exporting_variables)"
 
     if [ -n "${errors}" ]
     then