arm-bsp/fvp-base: set FVP_ variables

So that a fvp-base image can be tested easily, set the relevant FVP_*
variables.

After setting IMAGE_CLASSES += "fvpboot" in local.conf, this will show
the firmware start and Linux boot:

$ MACHINE=fvp-base bitbake core-image-base
$ runfvp fvp-base --console

Change-Id: Id1bd6cea57958117103e53ab9f78a58ce92b6a2e
Signed-off-by: Ross Burton <ross.burton@arm.com>