checkkconfigsymbols.py: improve detection of defects

This patch improves the detection of defects by updating the
regular expression to find Kconfig identifiers in the source
code, and fixes some cases of false positives. The following
changes are made:
- improve regex to find Kconfig identifiers in the source
- exclude .log files from analysis
- improve filtering of false positives (e.g, CONFIG_XXX)
- change output format from (feature:\tlist) to (feature\tlist)

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>