bbvars.py: add a script to look for undocumented variables

bbvars.py: add a script to look for undocumented variables

bbvars.py will compare recipes in meta directories with documentation files
and report on variables that don't appear to be documented. It reports the
number of times a variable is used as well as any doctags present in the
documentation config file.

The output of this is intended to aid in determining where documentation may
be lacking, but it is not perfect, and does generate some false positives. An
experienced eye and careful attention to count and doctag should be applied to
the results.

$ ./bbvars.py -d ../../documentation/poky-ref-manual/poky-ref-manual.html -m ../../meta -t ../../meta/conf/documentation.conf -T  | head -n 10
Found 1413 undocumented bb variables (out of 1578):
VARIABLE                              COUNT DOCTAG
===================================================
BUILD_ARCH                            4     The name of the building architecture. E.g. i686.
BUILD_CC_ARCH                         2     FIXME
BUILD_PREFIX                          4     FIXME
BUILD_SYS                             13    FIXME
BUILD_VENDOR                          2     FIXME
CACHE                                 1     The directory holding the cache of the metadata.
COMPATIBLE_HOST                       19    A regular expression which matches the HOST_SYS names supported by the package/file. Failure to match will cause the file to be skipped by the parser.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Richard Purdie <richard.purdie@linuxfoundation.org>