makefile/utils: Add a function for checking that a string is not empty.

[jcarrano]

Contribution description

A call to $(call ensure_value,x,y) will fail with message y if x is empty, and otherwise return x. This can be useto write more compact makefiles, while still producing friendly error messages.

Testing procedure

I should provide an automated test for this, in the meantime:

$ cd makefiles/util
$ make -f test-checks.mk test-ensure_value
test-ensure_value
$ make -f test-checks.mk test-ensure_value-negative
test-checks.mk:7: *** "This should fail".  Stop.

Issues/PRs references

Split from #10342.

[cladmi]

I will add a test so that it is run in murdock.

[cladmi]

I added a test and updated the behavior of your test to verify the returned value of ensure_value.
The test prints nothing in case of success.

It can be tested by replacing, in the Makefile, the tested target by the other: {,_negative}.
Verifying that the check tests ensure_value returned value can be done by replacing it in checks.mk.

On the naming, I would prefer having the directory named utils instead of using the singular form.
I still kept my script with your convention.

[jcarrano]

I renamed it to utils and squashed.

[cladmi]

Nice catch on the typo in the error message.

Codacy wants one line less in the README… you can inline fix it directly.
The commit titles mention makefile/util and makefiles/util, please update them to makefiles/utils at the same time.

I re-ran the tests and verified that they also detected the errors by tweaking the test

[jcarrano]

Done. Stupid codacy.

[cladmi]

Agreed with the implementation and tested too.