linuxboot: update linuxboot.sh script

Replace the term "u-root" with "busybox" in the comments since booting
of stage-2 kernel with busybox prompt is automated. Also update the
disclaimer message, since we no longer boot stage-1 linux kernel by
replacing the UEFI Shell binary.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I2a3126f127ad67aca6be6906881b4acd6761235f