scripts: make SoC field mandatory in board.yml

With the support of omitting SoC field for single SoC boards then there
is no reason to have this field optional.

Users are still able to use a short board name when building.

But always requiring SoC field in the board.yml paves the way for future
enhancements to the new hw model without having to do a second update
to a lot of board.yml files.

Such enhancement can be:
- Create Kconfig SoC selection and thereby remove need for Kconfig
  boilerplate code, and also thereby remove risk of naming
  inconsistencies.
- Extend board documentation, so that web doc provides option to filter
  all boards with a given SoC.

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

scripts/schemas/board-schema.yml

@@ -59,7 +59,7 @@ schema;board-schema:
                   required: true
                   type: str
     socs:
-      required: false
+      required: true
       type: seq
       sequence:
         - type: map
@@ -69,8 +69,6 @@ schema;board-schema:
               type: str
             variants:
               include: variant-schema
-    variants:
-      include: variant-schema
 
 type: map
 mapping: