samples: sysbuild: add hello world for multiple board targets

Add a new "hello world" sample that can be built for multiple board
targets. This sample can be used to see if Zephyr boots successfully
with _minimal_ configuration on SoCs with multiple cores, or boards with
multiple SoCs.

The following configurations are provided for now:

- nRF5340 DK: cpuapp+cpunet
- nRF54H20 DK: cpuapp+cpurad, cpuapp+cpuppr, cpuapp+cpuppr(xip)

Note that because of a quoting bug somewhere in the build system,
SB_CONFIG_REMOTE_BOARD is specified in .conf files (see the sysbuild/
directory). This should be fixed in the near future.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>