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 includes
the  stage-1 linuxboot kernel as a UEFI application. The build flag
$LINUXBOOT_BUILD_ENABLED is used to used to enable this feature. This is
an initial approach to support linuxboot for Neoverse reference design
platforms.

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