yocto/kas: Include FVP_* variables in env section

The FVP can be fetched from different URL depending on the
development phase, the current KAS configuration already
accepts FVP_BUILD_NUMBER to be passed through the environment,
so to offer more flexibility add the remaining variables that
allows the URL to be changed together with optional access control
variables for username and password.

While there, add also FVP_OVERRIDE, since the current configuration
allows to change the FVP build number, but without the former, the
recipe will complain on the binary SHA check.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>