sgi/build-test-busybox: move build-sgiboot script to sgi/build-test-busybox

The existing build-sgiboot.sh script which is used to prepare the binary
images for boot upto busybox shell is actually SGI specific. So move it
into 'sgi' platform folder and rename it as build-test-busybox.sh.

Change-Id: Iffa92f0d3565fbe5fccb28073b04d6cff95d9350
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>