testimage.bbclass, lib/oeqa/utils/qemurunner.py: make boot timeout configurable and...

testimage.bbclass, lib/oeqa/utils/qemurunner.py: make boot timeout configurable and increase it

On a loaded host (such as an AB setup where multiple builds run)
the 200 seconds timeout might not be enough to reach the login prompt.
Also make it configurable so we can set it from local.conf/auto.conf

(From OE-Core rev: e828571683333e6f18ed56a84e114b5e54975bc7)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>