build: namespace the generated headers with `zephyr/`

Namespaced the generated headers with `zephyr` to prevent
potential conflict with other headers.

Introduce a temporary Kconfig `LEGACY_GENERATED_INCLUDE_PATH`
that is enabled by default. This allows the developers to
continue the use of the old include paths for the time being
until it is deprecated and eventually removed. The Kconfig will
generate a build-time warning message, similar to the
`CONFIG_TIMER_RANDOM_GENERATOR`.

Updated the includes path of in-tree sources accordingly.

Most of the changes here are scripted, check the PR for more
info.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>