How to enable early printk

Early printk can only be enabled if CONFIG_DEBUG=y or in EXPERT mode. You
may want to enable it if you are debugging 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.

Select one of the "Early printk via * UART" in the choice possible for
"Early printk" in the "Debugging options" of Kconfig. You will then need to
set other options, which depends on the driver selected.

CONFIG_EARLY_UART_BASE_ADDRESS is a mandatory argument, it is the base
physical address of the UART to use.

Other options depends on the driver selected:
  - 8250
    - CONFIG_EARLY_UART_8250_REG_SHIFT is, optionally, the left-shift to
      apply to the register offsets within the uart.
  - pl011
    - CONFIG_EARLY_UART_PL011_BAUD_RATE is, optionally, a baud rate which
      should be used to configure the UART at start of day.

      If CONFIG_EARLY_UART_PL011_BAUD_RATE  is set to 0 then the code will
      not try to initialize the UART, so that bootloader or firmware
      settings can be used for maximum compatibility.
  - scif
    - CONFIG_EARLY_UART_SCIF_VERSION_* is, optionally, the interface version
      of the UART. Default to version NONE.

  - For all other uarts there are no additional options.

As a convenience it is also possible to select from a list of
predefined configurations available in the list of choice for "Early
printk" for specific platform.

By default early printk is disabled.
