-Wundef - no default for OPENTHREAD_ENABLE_VENDOR_EXTENSION/OPENTHREAD_EXAMPLES_SIMULATION

[tima-q]

Describe the bug
When using the -Wundef flag the following flags show up to have no default:

• OPENTHREAD_ENABLE_VENDOR_EXTENSION
• OPENTHREAD_EXAMPLES_SIMULATION
• OPENTHREAD_CONFIG_PLATFORM_BOOTLOADER_MODE_ENABLE

Blocks using -Werror in a build + currently these flags need to be added as dummy 0 in a config .h

To Reproduce Information to reproduce the behavior, including:

1. Was added a very long time ago
2. IEEE 802.15.4 hardware platform - any
3. Build steps - normal build with none of the mentioned flags in the configuration header
4. Network topology - NA

Expected behavior
Build does not throw 'undefined' warning

Additional context
There is no default for the mentioned flags defined in code. The define is only generated in CMake options - also no fallback there to 0. Adding a #if !defined construction as done for other config's in the common headers would fix this

[thcu-gp]

Is it an option to enable this -Wundef by default with warnings as errors so these get fixed immediately when new flags are added?