sys/fido2: improve & simplify flash handling

[Ollrogge]

Contribution description

This PR tries to improve the flash handling done by the FIDO2 implementation. Specifically this entails:

• Usage of mtd_aux device to reserve flash memory for the FIDO2 data
• No more usage of functionality that requires flash to be memory-mapped (e.g. _flash_is_erased function)
• Coherent usage of the mtd_flashpage api

Since the AUX slot approach is integrated into the mtd API, which is the recommended method for working with flash, I wonder if the periph_flashpage_in_address_space feature I added to reserve flash memory for FIDO2 can now be deprecated ?

@benpicco, with the mtd_aux approach, how can two different applications reserve flash memory without risking corruption of each other's data? Is it possible to divide the AUX slot into multiple smaller slots, that can be allocated for use by different applications?

Testing procedure

• /tests/sys/fido2_ctap with both native and nrf52840dk as target
• /tests/sys/fido2_ctap_hid