Add the environment variable CENGN_STRATEGY for supporting CENGN.
Set the default value to 'cengn_first', and we also can set the value
to the other three value 'cengn', 'upstream' or 'upstream_first'.
Story: 2008862
Task: 44361
Signed-off-by: Zhixiong Chi <zhixiong.chi@windriver.com>
Change-Id: I0326da798e808dce1b6fefd49e54f54ca41e8bcd