sanitycheck: support requirements on env variables

Some boards depend on environment variables so we want to make sure we
do not attempt to build boards requiring additional setup.

Add the section below into the board YAML file, sanitycheck will check
the environment and will only run tests on that board if the variables
are defined.

env:
  - VAR1
  - VAR2

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

scripts/sanity_chk/sanitycheck-platform-schema.yaml

@@ -28,6 +28,11 @@ mapping:
     seq:
       -
         type: str
+  "env":
+    type: seq
+    seq:
+      -
+        type: str
   "ram":
     type: int
   "flash":