ANDROID: dm: mount as linear target if eng build

eng builds dont have verity enabled i.e it does even
have verity metadata appended to the parition. Therefore
add rootdev as linear device and map the entire partition
if build variant is "eng".

(Cherry-picked based on
https://partner-android-review.git.corp.google.com/#/c/618690/

)

BUG: 29276559
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: I8f5c2289b842b820ca04f5773525e5449bb3f355