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.
- pull in headers for essential modules
- init clocks, gpios, uart0 out of platform_early_init
- wire up debug getc/putc to uart0
- comment out "native" uart impl
- 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()
