efi: Add shared printk wrapper for consistent prefixing

Add a wrapper for printk to standardize the prefix for informational and
error messages from the EFI stub.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>