arch/x86: Use EFI console as default printk handler

Where we have access to a bootstrap UEFI environment, it's productive
to use that console as the default printk handler.  That avoids the
bringup hassle of trying to configure UART settings blindly, as has
been customary.  It also emits nice text to the framebuffer on devices
with no serial port or other debug harness at all.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>