Fix printf fault before UART was initialized
Fix printf fault before UART was initialized

This fixes an issue were `os_default_irq` is trying to print dump
information before UART initialization was completed, causing a fault
and losing all stack/backtrace data, making debug impossible.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core