features/reset_to_bl31: fix the boot command

The document previously stated that the command was to build for Busybox
boot but the command was actually for uefi. Corrected the command to
build for Busybox boot.

Also added a line to clearly specify how to boot the system after
building.

Change-Id: I0c9737411b422edbff36156f6e1fa0eb56a87317