dist/tools/buildsystem_sanity_check: check no PORT exports

[cladmi]

Contribution description

Add sanity check for removed exports.

PORT is not exported anymore since #10440 so prevent for re-appearing.

Testing procedure

./dist/tools/buildsystem_sanity_check/check.sh

Returns no errors. (It is executed by the static tests too so would detect issues).

If adding this diff:

diff --git a/Makefile b/Makefile
index 144cbd7e6..fe51b1bda 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,9 @@
 .all:
+# Nothing after: export PORT
+export PORT = space
+export PORT?= question_mark
+export PORT= equal
+export PORT:= colon
 
 .PHONY: all doc doc-man doc-latex docclean print-versions welcome

All the lines are detected

./dist/tools/buildsystem_sanity_check/check.sh 
Invalid build system patterns found by ./dist/tools/buildsystem_sanity_check/check.sh:
Variables must not be exported:
	Makefile:# Nothing after: export PORT
	Makefile:export PORT = space
	Makefile:export PORT?= question_mark
	Makefile:export PORT= equal
	Makefile:export PORT:= colon

Issues/PRs references

Follow-up to #10440 and part of #10850

[miri64]

ACK. Tested by applying the following patch:

diff --git a/makefiles/tools/serial.inc.mk b/makefiles/tools/serial.inc.mk
index 215a5057e8..bad2d95f85 100644
--- a/makefiles/tools/serial.inc.mk
+++ b/makefiles/tools/serial.inc.mk
@@ -1,9 +1,9 @@
 # set default port depending on operating system
 OS := $(shell uname)
 ifeq ($(OS),Linux)
-  PORT ?= $(PORT_LINUX)
+  export PORT ?= $(PORT_LINUX)
 else ifeq ($(OS),Darwin)
-  PORT ?= $(PORT_DARWIN)
+  export PORT ?= $(PORT_DARWIN)
 endif
 ifeq ($(PORT),)
   $(info Warning: no PORT set!)

$ ./dist/tools/buildsystem_sanity_check/check.sh 
Invalid build system patterns found by ./dist/tools/buildsystem_sanity_check/check.sh:
Variables must not be exported:
	makefiles/tools/serial.inc.mk:  export PORT ?= $(PORT_LINUX)
	makefiles/tools/serial.inc.mk:  export PORT ?= $(PORT_DARWIN)
$ echo $?
1

When reverting the patch the script passes without any output and status code 0

[cladmi]

Thank you for the review.