Platform/Sgi: Add LinuxBoot support

LinuxBoot is a firmware that replaces specific firmware functionality
like the UEFI DXE phase with a Linux kernel and runtime.

This patch adds LinuxBootPkg and SgiPlatformLinuxBoot.fdf which replaces
the shell application with stage-1 linuxboot kernel and will be built
when build flag $LINUXBOOT_BUILD_ENABLED is set. The stage-1 linuxboot
kernel uses the same GUID as the shell application, enabling the user to
boot the stage-1 linxuboot kernel when launching the Shell from the Boot
Manager.

This is an initial implementation of linuxboot for RD platforms and not
the final version.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I952bd76487c9a643e39cd82ddcb222417a883847