Bug#741438: Initialize Apple graphics muxer when booting from GRUB

[Thibaut Paumard]

Control: tags -1 + patch

Hi,

The attached patch:

 - adds support for five variables in grub-mkconfig:
   GRUB_APPLE_GMUX_INIT
   GRUB_APPLE_GMUX_SELECT
   GRUB_APPLE_GMUX_DISPLAY
   GRUB_APPLE_GMUX_DDC
   GRUB_APPLE_GMUX_POWER

 - makes use in these variables in 10_linux

 - fills some default in /etc/default/grub. When GRUB_APPLE_GMUX_INIT
   is "yes", sets some other variables that are needed in this context:

   GRUB_PRELOAD_MODULES: without it, the screen remains blank until the
   computer boots (the user never sees the grub menu).

   GRUB_GFXPAYLOAD_LINUX=console: without it, the screen remains black
   in console mode, i.e. until X11 starts.

   GRUB_CMDLINE_LINUX_DEFAULT: add several options, without which the
   screen also remains black.

All of these are *necessary* to get recent Apple Macbook pros to display
anything when boot in EFI mode, and I also believe that these setting
are *sufficient* for the purpose.

I think it would be really nice to let the users do all of that with
just uncommenting one line instead of tinkering like I did for several
weeks...

Ideally, it would be great to have grub-mkconfig detect automatically
whether to enable GRUB_APPLE_GMUX_INIT, however I don't know how this
could be done. As is, the user will still need to access his box through
ssh or boot from the install medium to fix this setting.

Kind regards, Thibaut.