arm/fvpboot: add class to write FVP boot configuration

The fvpboot class is used to write a machine-readable file that describes
how to boot a given machine image inside a FVP, similar to how qemuboot
writes files for runqemu.

BSPs should set the FVP_* variables as appropriate in their machine
configuration, but let the user opt-in to adding fvpboot to IMAGE_CLASSES.

Change-Id: I2d1cd86ec4affc1d688a293987890d6a89124d94
Signed-off-by: Ross Burton <ross.burton@arm.com>