Fix asserts in developer images

:Release Notes:
Asserts should be disabled only in Mass Production builds,
when WEBOS_DISTRO_PRERELEASE == "".

:Detailed Notes:
Previously asserts was enabled for "devel" phase of the project.
This was changed to enable release in all development images.
Now asserts are disable only in Mass Production builds
where WEBOS_DISTRO_PRERELEASE == "".

:Testing Performed:
Builds was tested with an appropriate values.

:QA Notes:

:Issues Addressed:
[GF-43805] LS2: Fix asserts in development images.

Open-webOS-DCO-1.0-Signed-off-by: Alexandr Smoktal <alexandr.smoktal@lge.com>

Change-Id: I6fd295e59875231fc6f3c4b48bddb0bc1f04bdbc
Reviewed-on: https://g2g.palm.com/3721
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Tested-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>