[Firmware] Implement mcu-nrf52840 such that things compile.

[TheButlah]

We should add a new feature flag called mcu-nrf52840 to the existing mcu-* feature flag set. If active, we will not have any esp-hal or esp-wifi dependencies, and will only have nrf-hal related stuff.

To make matters easier, I suggest we start by using defmt-rtt as the global logger, and net-stubbed as the network transport. This will ensure that we only need to focus on getting the right setup done for the HAL, passing it to the imu driver, configuring the right feature flags, and setting up embassy.