1. A method for rebooting system software for a computer system after updating a boot image, the method comprising: reloading a boot loader module for the system software into system memory; shutting down all processing units of the computer system, except a selected one to be used for rebooting; and executing the reboot operation on the selected processing unit to reboot the system software, wherein the reboot operation does not run platform firmware of the computer system and includes the steps of: loading one or more updated modules of the boot image into the system memory; executing the boot loader module to load for execution modules of the boot image including a system software kernel and the updated modules; and transferring execution control to the system software kernel.