Document some recently added tasks and variables.
(From OE-Core rev: f650d584bf446f5923a40ca9a39ddcbbf31bf886)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>