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.
Helps with bare metal debugging, as we can't be sure our implementation
will work with a given machine.
As reported by someone on Discord, their machine hangs when we attempt
the dummy transfer.
