snippets: initial snippet.yml support

Add a new script, snippets.py, which is responsible for searching
SNIPPET_ROOT for snippet definitions, validating them, and informing
the build system about what needs doing as a result.

Use this script in snippets.cmake to:

- validate any discovered snippet.yml files
- error out on undefined snippets
- add a 'snippets' build system target that prints all snippet
  names (analogous to 'boards' and 'shields' targets)
- handle any specific build system settings properly,
  by include()-ing a file it generates

With this patch, you can define or extend a snippet in a snippet.yml
file anywhere underneath a directory in SNIPPET_ROOT. The snippet.yml
file format has a schema whose initial definition is in a new file,
snippet-schema.yml.

This initial snippet.yml file format supports adding .overlay and
.conf files, like this:

  name: foo
  append:
    DTC_OVERLAY_FILE: foo.overlay
    OVERLAY_CONFIG: foo.conf
  boards:
    myboard:
      append:
        DTC_OVERLAY_FILE: myboard.overlay
        OVERLAY_CONFIG: myboard.conf
    /my-regular-expression-over-board-names/:
      append:
        DTC_OVERLAY_FILE: myregexp.overlay
        OVERLAY_CONFIG: myregexp.conf

(Note that since the snippet feature is intended to be extensible, the
same snippet name may appear in multiple files throughout any
directory in SNIPPET_ROOT, with each addition augmenting prior ones.)

This initial syntax aligns with the following snippet design goals:

- extensible: you can add board-specific support for an existing
  snippet in another module

- able to combine multiple types of configuration: we can now apply a
  .overlay and .conf at the same time

- specializable: this allows you to define settings that only apply
  to a selectable set of boards (including with regular expression
  support for matching against multiple similar boards that follow
  a naming convention)

- DRY: you can use regular expressions to apply the same snippet
  settings to multiple boards like this: /(board1|board2|...)/

This patch is not trying to design and implement everything up front.
Additional features can and will be added to the snippet.yml format
over time; using YAML as a format allows us to make
backwards-compatible extensions as needed.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

scripts/schemas/snippet-schema.yml

@@ -0,0 +1,45 @@
+# SPDX-License-Identifier: Apache-2.0
+#
+# Copyright (c) 2022, Nordic Semiconductor ASA
+
+# A pykwalify schema for basic validation of the snippet.yml format.
+
+schema;append-schema:
+  # Sub-schema for appending onto CMake list variables.
+  # See uses under 'append:' keys below.
+  type: map
+  mapping:
+    DTC_OVERLAY_FILE:
+      type: str
+    OVERLAY_CONFIG:
+      type: str
+
+type: map
+mapping:
+  name:
+    required: true
+    type: str
+  append:
+    example: |
+      Snippet-wide appending can be done here:
+
+        name: foo
+        append:
+          DTC_OVERLAY_FILE: m3.overlay
+    include: append-schema
+  boards:
+    example: |
+      Board-specific appending can be done here:
+
+        name: foo
+        boards:
+          qemu_cortex_m3:
+            append:
+              DTC_OVERLAY_FILE: m3.overlay
+    type: map
+    mapping:
+      regex;(.*):
+        type: map
+        mapping:
+          append:
+            include: append-schema