new global_logger, used in AVR_LOG(), default is stdout/stderr
refactor logger
Merge pull request #12 from ponty/logger2

new global_logger, used in AVR_LOG(), default is stdout/stderr
Revert "Merge pull request #12 from ponty/logger2"

This reverts commit 4a345c29fe62858693a226a3f6adbef1021236b1, reversing
changes made to 77c0525fa53ac81daaf277593767cc5659e2e43e.
core: Add an overridable logging function

AVR_LOG() now uses a logging function which can be overridden as
necessary. By default, it logs to stdout/stderr.

Fixed-up-by: schuay <jakob.gruber@gmail.com>