API for storing arbitrary application data on the SD card

[prusnak]

Let's create API for storing arbitrary application data on the SD card (supersedes #574 + #323).

[prusnak]

Andrew raised some good points in #783, I am closing that PR, but don't want these to get lost, so I am posting them here:

• [ ] https://github.com/trezor/trezor-firmware/pull/783#discussion_r363335717
• [ ] https://github.com/trezor/trezor-firmware/pull/783#discussion_r363374849