configs/arm_foundationv8: fix build and update documentation

The arm_foundationv8_defconfig currently doesn't build because it uses
an external AArch64 toolchain that has been updated (Linaro toolchain)
and is now based on gcc 5.x. Unfortunately, Linux 3.13 is too old to
be built with gcc 5.x.

As a minimal fix to make this defconfig build again, we switch to use
the internal toolchain backend. At the time this defconfig was
introduced, there was no support for AArch64 in the internal toolchain
backend, but now that it is available, there is no reason to not use
it. This makes the defconfig also more consistent with the other
defconfigs.

Obviously, the kernel headers used to build the toolchain are set to
the fixed version 3.13, so that they match the kernel being used.

While we're at it, the readme.txt file corresponding to this defconfig
is updated. Runtime testing has been performed with the latest version
of the ARM Foundation model.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>