configs/sgi575: update the grub menuentry name

Update the grub menu entry name as "SGI-575 Busybox". This indicates
the platform on which the grub loads the next stage of software.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I8030a3d3d331028d2bf6d4cb5039c0ab32a0ccc8