sysbuild: use ARGN instead of ARGV1

Use of ARGV1 is undefined when number of arguments to function is less
than 2. Therefore switch to ARGN which holds arguments beyond required
arguments.
If there are no optional arguments, then ARGN is just an empty list,
thus making it safe to use.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>