Example Pico builds working pw_sys_io_pico

- Added pico_core_sources and pico_executable
- pico_base toolchain for building bs2_default.elf
- pw_sys_io_pico hooked up but not working in blinky yet
  Still need to call pw_pico_Init() on start
- pw_sys_io_pico over usb serial works
  But blinky/main.cc needs to manually call pw_sys_io_pico_Init();

Change-Id: I5b2c2e5a2528a0b36c7572b5921d781b6eb811ff