fix(fvp): work around BL31 progbits exceeded

fix(fvp): work around BL31 progbits exceeded

It is useful to have a single build for the FVP that includes as much
stuff as possible. Such a build allows a single TF-A build to be used on
a wide variety of fvp command lines. Unfortunately, the fvp also has a
(somewhat arbitrary) SRAM limit and enabling a bunch of stuff overruns
what is available.

To workaround this limit, don't enable everything for all
configurations. The offending configuration is when tsp is enabled, so
try to slim the binary down only when building with it.

As this doesn't solve the issue of running out of space for BL31, update
the linker error to give some clue as to what has (likely) caused it
while more permanent fixes are found.

Also add FEAT_RNG to the mix as it got missed in the commotion.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Icb27cc837c2d90ca182693e9b3121b51383d51fd