utils/check-symbols: new script

This script checks for inconsistencies on symbols declared in Config.in
and used in .mk files.
Currently it checks only symbols following the pattern BR2_\w+ .

The script first gets the list of all files in the repository (using git
ls-files like 'make check-flake8' already do).

Then it parses all relevant files, searching for symbol definitions and
usages, and add entries into a database.

At the end, the database is searched for inconsistencies:
- symbol that is part of "choice" and is referenced with "select";
- legacy symbol being referenced in packages;
- legacy symbol being redefined in packages;
- symbol referenced but not defined;
- symbol defined but not referenced;
- legacy symbol that has a Note stating it is referenced by a package
  (for legacy handling) but is referenced in the package without a
  comment "# legacy";
- legacy symbol that has a Note stating it is referenced by a package
  but it is not actually referenced.

There is also a debug parameter --search that dumps any filename or
symbol entries from the database that matches a regexp.

Sample usages:
$ utils/check-symbols
$ utils/docker-run utils/check-symbols
$ utils/check-symbols --search 'GETTEXT\b|\/openssl'

At same time the script is created:
- add unit tests for it, they can be run using:
  utils/docker-run python3 -m pytest -v utils/checksymbolslib/
- add two more GitLab CI jobs: check-symbols (to check current tree
  using the script) and check-check-symbols (to check the script against
  its unit tests)

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
[Peter: print warnings to stderr, rename change_current_dir() to
	change_to_top_dir()]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/generate-gitlab-ci-yml

@@ -26,7 +26,7 @@ gen_tests() {
     local do_basics do_defconfigs do_runtime do_testpkg
     local defconfigs_ext cfg tst
 
-    basics=( check-package DEVELOPERS flake8 package )
+    basics=( check-package check-symbol DEVELOPERS flake8 package symbol )
 
     defconfigs=( $(cd configs; LC_ALL=C ls -1 *_defconfig) )