arm-trusted-firmware: add option to enable/disable building FIP image

arm-trusted-firmware: add option to enable/disable building FIP image

Currently, our arm-trusted-firmware unconditionally builds a FIP
(Firmware Image Package). While this is often needed on platforms
where ATF encapsulates U-Boot, it is not the case on some other
platforms where it's U-Boot that encapsulates parts of ATF.

In order to prepare the support for the later platforms, we make
building the FIP image optional, and update the only defconfig we have
that uses ARM Trusted Firmware.

Note: we considered adding a "default y" here to preserve backward
compatibility, but there really isn't any default that is sane:
whether a FIP image needs to be built or not is purely platform
specific.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>