pbio/platform/prime_hub: Enable slots.

[laurensvalk]

Support for basic multi-program slots has been added a while ago, but it is not enabled by default while we wait on related featured to be completed.

This enables it for those who wish to try it out early.

Try it out

• Download the Prime Hub firmware file using the link in the post below.
• Install that file using these instructions.
• Code as usual using https://beta.pybricks.com/
• On the hub, choose slot 1 to 5 using the left/right buttons.
• A new program is downloaded to the selected slot.

Caveats

• There is no file manager yet. To delete programs, just download an empty program to that slot.
• Upgrading to a new firmware will erase the programs from the hub.
• All programs are kept in memory at once. If you write 5 giant programs, it will have a bit less memory remaining to run them.

[github-actions]

Download the artifacts for this pull request:

• pr_number.zip
• mpy-cross.zip
• nxt-firmware-build-3647-git7f1b7f84.zip
• pybricks-micropython-build-3647-git7f1b7f84.zip
• movehub-firmware-build-3647-git7f1b7f84.zip
• cityhub-firmware-build-3647-git7f1b7f84.zip
• technichub-firmware-build-3647-git7f1b7f84.zip
• primehub-firmware-build-3647-git7f1b7f84.zip
• essentialhub-firmware-build-3647-git7f1b7f84.zip

[coveralls]

coverage: 56.36%. remained the same
when pulling e37a731 on slot
into cc5521f on master.