dbus: Add support for vendor specific elements

dbus: Add support for vendor specific elements

The new methods are
1. VendorElemAdd "i" "ay" i=integer ay=array of bytes
2. VendorElemGet "i" i=integer (output array of bytes)
3. VendorElemRem "i" "ay" i=integer ay=array of bytes

These provide functionality similar to the control interface commands
VENDOR_ELEM_ADD, VENDOR_ELEM_GET, and VENDOR_ELEM_REMOVE.

Signed-off-by: Avichal Agarwal <avichal.a@samsung.com>
Signed-off-by: Purushottam Kushwaha <p.kushwaha@samsung.com>
Signed-off-by: Kyeong-Chae Lim <kcya.lim@samsung.com>
Signed-off-by: Mayank Haarit <mayank.h@samsung.com>
Signed-off-by: Dilshad Ahmad <dilshad.a@samsung.com>
[VendorElemGet to return array of bytes instead of string; cleanup]
Signed-off-by: Jouni Malinen <j@w1.fi>