Add WEBOS_INSTALL_DB8MEDIADIR variable

:Release Notes:
Add the install location of the db8 media directory.

:Detailed Notes:

:Testing Performed:
Compiled and tested locally.

:QA Notes:

:Issues Addressed:
[GF-51737] db8: Unhide directory /media/cryptofs/data/.db8

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <maxim.sditanov@lge.com>
Change-Id: I94a5fa6baacde6e40084d9dc8947290dbe297e61
Reviewed-on: https://g2g.palm.com/3959
Reviewed-by: DCO Verification
Reviewed-by: Herb Kuta <herb.kuta@lge.com>
Tested-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>