RP2040 fixes

[kilograham]

Fix some edge case bugs in the RP2040 port

Description

1. malloc was not set to be thread safe by default when using RTOS when not using pico_multicore (i.e. compiling application for a single core only)
2. RP2040 port initialization with non static memory allocation caused IRQs to be disabled
3. Use of Raspberry Pi Pico SDK threading primitives or timers prior to FreeRTOS scheduler initialization caused a crash when not using pico_multicore (i.e. compiling application for a single core only)

[codecov]

Codecov Report

Merging #424 (fad53d1) into main (4c4089b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #424   +/-   ##
=======================================
  Coverage   92.15%   92.15%           
=======================================
  Files           4        4           
  Lines        1274     1274           
  Branches      343      343           
=======================================
  Hits         1174     1174           
  Misses         53       53           
  Partials       47       47           

Flag	Coverage Δ
unittests	92.15% <ø> (ø)

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