runfvp: pass arbitrary options after -- to the FVP binary

To allow passing arbitrary options to the FVP binary, split the passed
options on --.  Anything before the separator is handled by runfvp,
anything afterwards is passed as-is to the FVP binary.

Change-Id: I686b2fb79d217e26988753be7bd067c638d69eac
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>