cmake: rework empty zephyr libraries handling.

This is an update to #34289.

When using CMake <=3.18 an interface library may not use the property
SOURCES.

Therefore, if an interface lib is added to the list of ZEPHYR_LIBS, then
CMake <=3.18 will raise the following error:
```
CMake Error .... (get_property):
  INTERFACE_LIBRARY targets may only have whitelisted properties.
  The property "SOURCES" is not allowed.
```

Therefore the check has been reworked into two steps.
First ensure all libs are static, and if they are, then check if they
are empty and not imported.

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