bitbake.conf: add full stop to default DESCRIPTION

bitbake.conf: add full stop to default DESCRIPTION

SUMMARY should not end with a full stop; however if DESCRIPTION is not
set in a recipe and thus defaulted from SUMMARY, the additional
DESCRIPTION values for other standard packages e.g. ${PN}-dev look a bit
odd without a full stop separating the SUMMARY value and the rest of the
text. Add a full stop to avoid this.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>