This commit implements the `_thread` module on the zephyr port.
Due to the fact that we are still using a rather old version of Zephyr,
`CONFIG_DYNAMIC_THREAD` is not available and therefore the stack for
threads cannot be allocated dynamically, only at compile time. So for the
time being and for the purpose of this commit, a maximum of 4 Zephyr
threads (besides the main thread) can be created. Once we manage to update
to the latest version of Zephyr this won't be a problem anymore.
Configuration for the nrf52840dk is added as part of this change, because
this board was used to test the threading support.
The Zephyr option `CONFIG_THREAD_CUSTOM_DATA` is used to enable threading
on a per board basis. The `thread.conf` file is added as a convenient way
to enable threading.
Signed-off-by: danicampora <danicampora@gmail.com>
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