Add --pico-family parameter

[infiniteloop5]

Compilation of the pico-blink example failed with the following error:

usage: macho2uf2.py [-h] --base-address BASE_ADDRESS --segments SEGMENTS --pico-family PICO_FAMILY input output
macho2uf2.py: error: the following arguments are required: --pico-family

Looks like the --pico-family parameter became mandatory.

This PR adds a PICO_FAMILY variable near the top of the build script, with a comment pointing out the two possible values (according to the source code of macho2uf2.py).
It then then sets --pico-family to this variable.

Maybe it makes sense to set up some kind of CI to catch these kind of build errors automatically.

[iCMDdev]

I think you could also change the target architecture from armv6m to armv7m based on this for RP2350 boards, even though the latter is backwards compatible with the former (and should be fine as long as the swift binary isn't being linked to armv7m libs - this was the case with the Pico SDK examples, but was fixed in the last commit).

Maybe the slightly improved instruction set will bring some (perhaps tiny) performance benefits? 😁

[rauhul]

Sorry I missed this (was out sick) I think we can merge this as is!