arm/fvp-rd-kronos: Introduce the FVP_OVERRIDE variable

The FVP_OVERRIDE is used to disable bitbake from validating the FVP package's
and licenses' checksum.
This is useful to validate temporary FVP packages.

While there, remove the FVP_SERVER_URL variable check since it will always
have a default value set.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>