How to enable early printk

Early printk can only be enabled if debug=y. You may want to enable it if
you are debbuging code that executes before the console is initialized.

Note that selecting this option will limit Xen to a single UART definition.
Attempting to boot Xen image on a different platform *will not work*, so this
option should not be enable for Xens that are intended to be portable.

CONFIG_EARLY_PRINTK=mach
where mach is the name of the machine:
  - vexpress: printk with pl011 for versatile express

By default early printk is disabled.