printk: Refactor to allow passing a parsing context around

Pass a parsing context around, allowing counting the number of
characters written, which is then returned in the return value of
printk(). This makes printk more similar to printf and prepares the
way to extend the implementation to support printing to strings.

Change-Id: Ib28a18a4f36fc58b98b228fd5763b2c05f5af7bc
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>