[script] update the check-size report (#9368)

This commit updates the `check-size` script and how the OpenThread
size report is generated and reported.

The size report now includes four device types:
- FTD (not acting as a BR)
- MTD (including SED)
- Border Router (BR)
- RCP

Each type uses its own example config header file (e.g., BR uses
`examples/config/ot-core-config-check-size-br.h`). These header files
specify all the OT configs and enable/disable the set of features
that make sense for the given type.

This replaces the previous model where the same set of configs were
used for all types. This change allows us to track the code size of a
typical BR build, as well as the code size of other device types.

In order to build and generate a size report for the BR configuration
example with BR-specific features enabled (such as Border Routing
Manager or NAT64), we need an implementation of the related platform
APIs that are used by these features (e.g. `otPlatInfraIf` APIs).
This commit adds mock empty implementations of these APIs, which are
only included in the size-report builds.

script/test

@@ -656,7 +656,7 @@ main()
 {
     envsetup "$@"
 
-    if [[ -z ${1:-} ]]; then
+    if [[ -z ${1-} ]]; then
         print_usage 1
     fi