scripts: list_hardware: Find all SoCs before checking runner configs

When validating the flash runner configurations in `soc.yml`, the only
SoCs that were considered had to be defined under this structure:

   family:
     - series:
         - socs:
             - name: ...

However, the `family` and `series` keys are optional, so the `soc.yml`
files can also be arranged like this:

   family:
     - socs:
         - name: ...
   series:
     - socs:
         - name: ...
   socs:
     - name: ...

The solution is to move the validation code further down, so that it can
reuse the SoC data that was already correctly parsed while initializing
a `Systems` instance.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>