The zephyr build system supports merging application-level board
configurations, so there is no need to reproduce this functionality in
MicroPython.
If CONF_FILE is not explicitly set, then the zephyr build system looks for
prj.conf in the application directory. Therefore we rename the MicroPython
prj_base.conf to prj.conf.
Furthermore, if the zephyr build system finds boards/$(BOARD).conf in the
application directory, it merges that configuration with prj.conf.
Therefore we rename all the MicroPython board .conf files and move them
into a boards/ directory.
The minimal configuration, prj_minimal.conf, is left in the application
directory because it is used as an explicitly set CONF_FILE in
make-minimal.
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