Add a rust-based NVMC driver for the ATSAM4L8-XPRO board

[nihalpasham]

In order to port rustBoot to the ATSAM4L8-XPRO board, we'll need to implement a flash-driver written in rust.

Sub-tasks:

• The flash driver must read, write and erase data-blocks from the device's internal flash memory.
• Finally, it must implement rustBoot's FlashInterface trait.

Additional notes:

• You only need to implement the methods (of FlashInterface trait), applicable to your board.

[UdayakumarHidakal]

Acknowledged

[imrank03]

Acknowledged.