diag: Add DCI support

This change adds DIAG Consumer Interface (DCI). This will help
Android user space applications use DIAG to send command requests
to modem and receive responses back. Until now, this was done using
tools or scripts on PC. With this change, this functionality is available
on target itself.

Change-Id: I66c187e406fe93ae23c4ca0ead2cf0aed3502197
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>