testimage.bbclass: write testresult to json files

testimage.bbclass: write testresult to json files

As part of the solution to replace Testopia to store testresult,
OEQA testimage need to output testresult into json files, where
these json testresult files will be stored into git repository
by the future test-case-management tools.

By default, json testresult file will be written to "oeqa"
directory under the "WORKDIR" directory.

To configure multiple instances of bitbake to write json testresult
to a single testresult file at custom directory, user will define
the variable "OEQA_JSON_RESULT_DIR" with the custom directory for
json testresult.

(From OE-Core rev: 5d135d4769b6bb60d575eb6ed196367f9e077cc4)

Signed-off-by: Yeoh Ee Peng <ee.peng.yeoh@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>