Rp2040 Target Support

[gundralaa]

Basic unthreaded runtime support for rp2040 target.
Uses the pico-sdk as a dependency and utilizes the high level time library to provide timer support.

• changed semaphore util to lf_semaphore due to name collisions
• added lf_rp2040_support files
• modified platform cmake

[edwardalee]

This looks pretty good, but the functions defined need comments. E.g., _lf_clock_now should state that it return the time since boot in nanoseconds. Note that by convention, comments consist of full sentences with proper punctuation and javadoc style annotations. So, for _lf_clock_now, it should be:

/**
 * Write the time since boot in nanoseconds into the variable pointed to by the argument and return 0.
 * @param t Pointer to the variable to write to.
 */

Note the use of the imperative case above. I.e., don't say this:

• Writes the time since boot in nanoseconds into the variable pointed to by the argument and return 0.

The above is not a sentence in English.

Looks like all requested changes have been implemented.