Contribution description

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.

Testing procedure

No error when running the script as will CI check in static-tests.
No error when running shellcheck on the script.

Applying this diff will report a lot of errors as they are still exported.

diff --git a/dist/tools/buildsystem_sanity_check/check.sh b/dist/tools/buildsystem_sanity_check/check.sh
index e18ca6776..bd4071fd8 100755
--- a/dist/tools/buildsystem_sanity_check/check.sh
+++ b/dist/tools/buildsystem_sanity_check/check.sh
@@ -50,6 +50,8 @@ check_not_parsing_features() {
 UNEXPORTED_VARIABLES=()
 UNEXPORTED_VARIABLES+=('FLASHFILE')
 UNEXPORTED_VARIABLES+=('TERMPROG' 'TERMFLAGS')
+UNEXPORTED_VARIABLES+=('FFLAGS')
+UNEXPORTED_VARIABLES+=('PORT[ ?=]' 'PORT$')
 check_not_exporting_variables() {
     local patterns=()
     local pathspec=()

I used PORT here on purpose as it may need to be more complicated to not match PORT_LINUX too. Even if PORT_LINUX should not be exported anyway but another PORTSOMETHING may need to.

Issues/PRs references

Tracking: remove harmful use of export in make and immediate evaluation #10850
Useful for #10440