This allows forcing the use of only the framebuffer set up by the
bootloader and skips instantiating devices for any other graphics
cards that may be present.
The document describes the implications of enabling and disabling that
option on the ability to enable SMP mode, and describes the requirements
for enabling IOAPIC mode even without enabling SMP mode.
'bootmode' now only controls which set of services are started by
SystemServer, so it is more appropriate to rename it to system_mode, and
no longer validate it in the Kernel.
Bootmode used to control framebuffers, panic behavior, and SystemServer.
This patch factors framebuffer control into a separate flag.
Note that the combination 'bootmode=self-test fbdev=on' leads to
unexpected behavior, which can only be fixed in a later commit.
