Provides common Nvmc functionality

[huntc]

The embedded-storage traits are implemented to provide a higher-level abstraction for reading and writing from/to flash storage. In the first instance, support is provided for the nRF52 boards and the nRF9160, but other boards should be relatively easy to support if required in the future.

Sample usage of writing to flash given some 32 bit aligned slice of bytes:

// Erase a page
let _ = nvmc.try_erase(0, 4096);

// Write the 32 bit aligned slice
let _ = nvmc.try_write(0, some_u8_slice_on_32_bit_bounds);

An example of establishing the Nvmc on the nRF9160:

let mut nvmc = Nvmc::new(board.NVMC_NS, unsafe { &mut CONFIG });

...where CONFIG points to flash:

extern "C" {
    #[link_name = "_config"]
    static mut CONFIG: [u32; 1024];
}

...which can be expressed in a memory layout:

MEMORY
{
  /* NOTE 1 K = 1 KiBi = 1024 bytes */
  FLASH : ORIGIN = 0x00040000, LENGTH = 764K
  CONFIG : ORIGIN = ORIGIN(FLASH) + LENGTH(FLASH), LENGTH = 4K /* 4K is the flash page size */
  RAM : ORIGIN = 0x20020000, LENGTH = 128K
}

_config = ORIGIN(CONFIG);

Testing on the nRF52840 device can be achieved via cargo test --test nvmc from within the nrf52840-hal-tests folder.

Fixes #336

TODO:

• [x] Implement the ReadNorFlash methods
• [x] Implement the NorFlash methods
• [x] Provide an Instance for the nRF52840
• [x] Provide an Instance for the nRF9160
• [x] Provide a nvmc module in nrf52840-tests for the new abstraction
• [x] Write some more tests to test some boundary conditions
• [x] Go through the code finely and compare it again with the nrfx library
• [x] Provide an example of reading, writing and erasing from/to storage

[huntc]

https://github.com/titanclass/thingy91-lorawan-nbiot/pull/2/commits/4a9f6730e329758f4bc7a78be3f221466cbcfb81 shows the positive impact of this abstraction.

[huntc]

@jonas-schievink @Dirbaio We now have tests for the NVMC via nRF52840-hal-tests, and things appear to be working! I want to go through things finely before converting from a draft PR, and write an example. I also want to do some boundary checks via my tests. Your feedback is appreciated!

[huntc]

This PR is complete and ready for a thorough review.

[jonas-schievink]

bors r+

[bors[bot]]

Build succeeded:

• ci