Add ability to enable DHCPv6 Prefix Delegation (PD) of br omrprefix during build

[benbacon]

Checklist

• Checked the issue tracker for similar issues to ensure this is not a duplicate.
• Described the feature in detail and justified the reason for the request.
• Provided specific use cases and examples.

Feature description

Allow option to set br pd enable via menuconfig to enable DHCPv6 Prefix Delegation (PD) of Off-Mesh Routable (OMR) prefix at build.

Currently if it is desired to use an PD prefix as an alternative to the randomly generated Off-Mesh Routable (OMR) prefix it is necessary to execute commands at runtime to change this (and this configuration is lost upon restart).

Use cases

Would be useful for omrprefix to use a ULA/GUA prefix across restarts without requiring a physical connection to the device to re-enable; in a residential environment it is common for br devices to be unexpectedly powered off when powered in household sockets to ensure good Thread coverage so it is inconvenient to detect this has happened and to correct it.

Alternatives

No response

Additional context

Attempted to set additional compile definitions as below however output of br pd state at boot was disabled:

% git diff
diff --git a/examples/basic_thread_border_router/CMakeLists.txt b/examples/basic_thread_border_router/CMakeLists.txt
index 977822e..9910c8b 100644
--- a/examples/basic_thread_border_router/CMakeLists.txt
+++ b/examples/basic_thread_border_router/CMakeLists.txt
@@ -4,3 +4,4 @@ cmake_minimum_required(VERSION 3.5)

 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
 project(esp_ot_br)
+add_compile_definitions(OPENTHREAD_CONFIG_BORDER_ROUTING_DHCP6_PD_ENABLE=1 OPENTHREAD_CONFIG_BORDER_ROUTING_DHCP6_PD_CLIENT_ENABLE=1)

[chshu]

For any CLI command, there is a corresponding OpenThread API that you can call from your application. For example, you can use otBorderRoutingDhcp6PdSetEnabled() to enable the DHCPv6 PD feature.

[benbacon]

Thanks @chshu in the context of the basic_thread_border_router example should I just call this within examples/common/thread_border_router/src/border_router_launch.c ?