tools/qa-checks: Improve QA-check module configuration

This patch enables:
* Per-check configuration (and global defaults) to be provided via YAML file
* The contents of .gitignore to be easily passed to checks
* Gitstyle pattern matching supplied to checks (e.g. exclude or include files
  matching patterns like "*.log" or "/*artifacts*")
* Checks to define their required variables seemlessly, so that maintaining or
  extending a check module requires no changes to wrapper scripts.
* Fewer hard-coded variables defined in the check sources

This patch also adds in-source descriptions of the wrapper script and each
QA-check, fixes relative path handling, logging formatting inconsistencies, and
updates the checker outputs to record the number of files checked (to avoid
silently excluding all files with an unintentionally excessive pattern).

Issue-Id: SCM-3033
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Ib64fab9c1eee954febc7e5bf6e7a6b8ee26dab57