Add support for custom CMake flags and positional target arguments in build script

[schkovich]

• Refactor make-libpico.sh to accept positional arguments for target selection and a --flags option to forward custom CMake flags.
• Update environment variable exports for clarity.
• Add LIB_PICO_MULTICORE CMake option to CMakeLists.txt and propagate it to target compile definitions.
• Preserve backwards compatibility and improve build flexibility for local and CI workflows.

Example Usage:

/make-libpico.sh rp2040 --flags -DPICO_STDIO_UART=1 -DPICO_STDIO_USB=0 -DLIB_PICO_MULTICORE=1

Tested:

• Verified that the script works as before with no arguments.
• Confirmed that custom flags are correctly passed to CMake and reflected in the build.

Closes #3058

[maxgerhardt]

Settling LIB_PICO_MULTICORE=1 overlaps with work already done in #3049, I would wait for that to merge and then you can rebase your changes upon the latest version.

[earlephilhower]

Yes, this will probably be closed/superseded. We discussed it in #3058 and once we determined the flag was OK in general I just added it into the SDK update (since the libs will be rebuilt anyway).

[earlephilhower]

I think we're good with #3049 for this. If not, let's re-open something based off of the latest tag. Thx!