EFI: ignore EFI commandline, skip console setup when booted from GRUB

EFI: ignore EFI commandline, skip console setup when booted from GRUB

Update EFI code to completely ignore the EFI comnandline when booted from GRUB.
Previusly it was parsed of EFI boot specific options, but these aren't used
when booted from GRUB.

Don't do EFI console or video configuration when booted by GRUB.  The EFI boot
code does some console and video initialization to support native EFI boot from
the EFI boot manager or EFI shell.  This initlization should not be done when
booted using GRUB.

Update EFI documentation to indicate that it describes EFI native boot, and
does not apply at all when Xen is booted using GRUB.

Signed-off-by: Roy Franz <roy.franz@linaro.org>