configs: tdx: Do not overwrite fdtfile if it got set manually

In case a customer wants to set fdtfile currently preboot overrides it
always with preboot just before the bootdelay. Use test -n to check
if fdtfile is already set and only set it if nothing got touched manually

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
7 files changed