cppcheck: Move config files to build_tools

It's weird having them in the parent directory.

Move them to build_tools/ and make them visible.

[ci skip]

build_tools/cppcheck.sh

@@ -1,3 +1,6 @@
 #!/bin/sh
 
-cppcheck --enable=all --std=posix --quiet ./src/
+cppcheck --enable=all --std=posix --quiet \
+         --suppressions-list=build_tools/cppcheck.suppressions \
+         --rule-file=build_tools/cppcheck.rules \
+         ./src/