MLK-10136 imx_v7_defconfig: disable rfkill feature

MLK-10136 imx_v7_defconfig: disable rfkill feature

Currently wpa_supplicant in yocto rootfs does not work properly due to RFKILL
feature and Yocto upgrade.
It causes the WiFi in new Yocto rootfs unable to get ip address via DHCP.
The root cause is still unkown.
This patch temporarily disable RFKILL feature to make WiFi work first.

Current there's no user of RFKILL in Yocto rootfs.
We will enable it again if needed after the issue is fixed.

Signed-off-by: Dong Aisheng <b29396@freescale.com>