Enhancing cdboot [patch for review]

Hi,

Below please find patch that enhances cdboot with two compile-time options:

1. CDBOOT_SILENT. When this option is set, the cdboot doesn't produce
any messages except "Loading, please wait..." and it also passes
RBX_MUTE flag to the next stage to silence it as well. This is intended
for custom installations where end-user is not required to see any
messages or interfere with the boot process.

2. CDBOOT_PROMPT. When this option is enabled the cdboot behaves like
windows xp or vista cd loader, that is it reads MBR from the first hard
drive in the system and if the MBR is bootable (i.e. drive has some
other operating system installed on it) then it presents user with
"Press any key to boot from CD" prompt and waits 20 seconds. If key is
not pressed then the control is passed to the MBR, otherwise CD is
booted. This is intended for installation CD to allow unattended mode
and also helps when installation CD is unintentionally left in the drive
of the machine that is set to boot off CD.

Any comments/suggestions are appreciated. If there are no objections I
would like to commit the change. The long-term goal is to make
CDBOOT_PROMPT default mode for installation CD.

http://sobomax.sippysoft.com/~sobomax/cdboot.diff

-Maxim