tfa-next: Enable rfa-fvp run tests in LAVA CI

The rfa-fvp setup was done to be able to run tests locally.
Add the following changes so that the runtime (not just build)
rfa-fvp tests can also run on Jenkins CI and LAVA:

 * As all tfa-next tests require the modification of the RUSTUP_HOME
   variable, modify this directly in build_package.sh before
   pre_tf_build.
 * Generate the lava job template that is later used by gen_fvp_yaml.
 * Mirror the expect/tfa-next.exp file in expect-lava as needed by
   LAVA.

Change-Id: I6fd7a36227b402111d1b93f47803deee2575eb3f
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
diff --git a/run_config/fvp-next b/run_config/fvp-next
index 7b46cf0..3eb9802 100644
--- a/run_config/fvp-next
+++ b/run_config/fvp-next
@@ -5,6 +5,9 @@
 # SPDX-License-Identifier: BSD-3-Clause
 #
 
+generate_lava_job_template() {
+	payload_type="linux" gen_yaml_template
+}
 generate_lava_job() {
 	local model="base-aemv8a"