New boards with layouts for dual partition setups

[kaczmarczyck]

Adds the dev board layouts for a setup with a bootloader and 2 partitions.

Most of the changes in this PR are slightly changed copies of existing code. The nrf52840_layout_*.ld files are similar to the existing nrf52840_layout.ld, and the boards are similar to the non-partitioned variant, too. All changes from the existing dev board definition to the new ones are documented in the respective README.md.

Outside of boards and layouts, this biggest change is in the deploy script. Changes include:

• some refactoring
• paddings are optional
• new metadata writer

I'm not sure about the less redundant definitions of OpenSKBoard, please let me know what you think, I can revert them if you like. To have the boards running, you need to also flash the matching bootloader. Other than that, the deployment didn't change, just specify the board name.

[coveralls]

Coverage increased (+0.07%) to 42.059% when pulling 3a070a7 on kaczmarczyck:submit-upgrade into d085d54 on google:develop.

[jmichelp]

That's a lot of code being duplicated.
I wonder if we couldn't do better by passing the arguments through env variables and tweak the build.rs and the main.rs to go back to fewer boards (devkit, dongle, mdk).

[kaczmarczyck]

I reused the src/ from nrf52840dk_opensk. Now all 3 dev board definitions load the STORAGE_LOCATIONS from a generated locations.rs. I'm not sure about the benefit of completely removing board definitions. The cleanest solution for 3 different layouts should be 3 different layout.ld files. WDYT?