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Refactors the zephyr build infrastructure to build MicroPython as a
cmake target, using the recently introduced core cmake rules.

This change makes it possible to build the zephyr port like most other
zephyr applications using west or cmake directly. It simplifies building
with extra cmake arguments, such as specifying an alternate conf file or
adding an Arduino shield. It also enables building the zephyr port
anywhere in the host file system, which will allow regressing across
multiple boards with the zephyr twister script.
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>