fix realm distro boot test script

realm_distro_boot.sh runs on fvp docker so there is no scope
of setting Automate variable so remove it.

Also fvprun command won't exit so ored tee command seems to
not being executed, boot.sh already stores realm_distro_boot
test case logs to realm_distro_boot.log so use it to parse/wait
for debian boot.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>