archiver.bbclass: Improve the usability for the archiver classes

archiver.bbclass: Improve the usability for the archiver classes

The usability of the archiver classes can be improved, beyond the
simple addition of default values for the variables. A user could
well inherit just archiver rather than the individual useful classes,
and not realize it will do nothing.

[YOCTO #2472]

(From OE-Core rev: ce91f495e0c4ef3bf53ee8b2ea570061da38e14f)

Signed-off-by: Xiaofeng Yan <xiaofeng.yan@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>