Uses the target/debugconfig.h file pattern laid down before,
which is a bit long in the tooth but for the moment still helps
us separate target from platform.
- sys/cdefs is wanted for __CONCAT()
- version.h and config_autogen.h are generated by the pico sdk
- config_autogen expects to include board file
- instead define PICO_xyz in target rules.mk
- just a skeleton to get us started
- cpu feature config and irq names / vectab2 setup done
- pulled in the second stage for pico as a blob for now
- seems to be hard faulting out of systick setup in platform_init()
