warpboard: linux.fragment: Do not use CONFIG_BRCMFMAC as built-in

warpboard: linux.fragment: Do not use CONFIG_BRCMFMAC as built-in

Selecting the driver CONFIG_BRCMFMAC as built-in causes some probe issues
as we cannot guarantee that the brcmfmac driver will be probed after the
rootfs has been mounted. The brcmfmac driver retrieves the firmware and
nvram file from the rootfs, so the rootfs should be mounted first.

To avoid such issues let the CONFIG_BRCMFMAC be built as module, which is
the original option in imx_v6_v7_defconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>