arm-bsp: n1sdp: linux-linaro-arm: Disable CONFIG_EXTRA_FIRMWARE

We don't want to build firmware blobs into the kernel binary. Also,
add linux-firmware-rtl8168 package to MACHINE_ESSENTIAL_EXTRA_RDEPENDS
to have it installed in any image by default.

Change-Id: If8a88a4ccbc6d474438a2555498989695a4b19dc
Issue-Id: SCM-954
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>