Separate modchip code from the payload

[webhdx]

Right now PicoBoot firmware is distributed as a single *.uf2 file. This overwrites modchip code and payload that gets injected. I've always wanted to separate the two and let other projects release their payloads to be used with PicoBoot (https://github.com/OffBroadway/flippyboot-ipl, https://github.com/redolution/iplboot). I definitely don't want anyone to release uf2 file containing modchip code without mentioning the exact version of the code. This is what's happening with cubeboot today 😞

This is mostly needed for Pico W as we will need to store some connection data in the flash which would be overwritten by applying updates.

When #56 is implemented, separate *.uf2 file will overwrite whole LFS partition.

[webhdx]

#107 implements it. It still requires proper documentation.