scripts: add runfvp

runfvp is a script that reads existing .fvpconf files (as written by
fvpboot.bbclass) and executes the FVP binary appropriately.

By default the behaviour is the same as running a FVP by hand, but by
passing --console the first serial port is connected to stdin/stdout.

Change-Id: Ibd26867c09e8692be4c02a7ea13571dcfe36db9b
Signed-off-by: Ross Burton <ross.burton@arm.com>