Warning when building mcuboot child image for Aludel Elixir Rev A board

[cdwilson]

When building for aludel_elixir_ns@A, the following warning is printed when the mcuboot child image is being built:

warning: The choice symbol LTE_NETWORK_MODE_NBIOT (defined at
/Users/chris/workspace/cgnd/clients/golioth/golioth-reference-design-can-asset-
tracker/deps/nrf/lib/lte_link_control/Kconfig:166) was selected (set =y), but no symbol ended up as
the choice selection. See
http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_LTE_NETWORK_MODE_NBIOT and/or look up
LTE_NETWORK_MODE_NBIOT in the menuconfig/guiconfig interface. The Application Development Primer,
Setting Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be
helpful too.

I think this is because aludel_elixir_A.conf defines CONFIG_LTE_NETWORK_MODE_NBIOT=y (required because Rev A was built with NRF9160-SIBA-R7), but this symbol is not defined when building mcuboot.

Once this is fixed, reminder to update: zephyrproject-rtos/zephyr#67808