Store MAC address in lower case to vconf

Store MAC address in lower case to vconf

MAC address is provided in the following two ways.
  - file: /sys/class/net/wlan0/address
  - vconf: db/wifi/bssid_address

The former is always provided in lower case,
but the latter is provided in upper or lower case depending on the HAL.
This can affect services that generate ID using MAC address,
so the related code has been modified so that the vconf value is always
provided in lowercase.

Change-Id: If47a8d9f8728b5ec451c1542e7e24636f1cb64c2
Signed-off-by: Jaehyun Kim <jeik01.kim@samsung.com>