Add fbconsole USE flag for framebuffer console support.

Add fbconsole USE flag for framebuffer console support.

On ARM kernel, framebuffer console is turned off by default. But in recovery
and factory shims, we need it to output messages.

BUG=chromium-os:13607
TEST=add fbconsole USE flag when emerging kernel in mod_image_for_recovery.sh.
Boot to recovery image and see messages showed on framebuffer console.

Change-Id: Icbf9e57ba11535320250bfe0d6e5a940da99ea11

R=olofj@chromium.org,rongchang@chromium.org

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=47f2af3

[Olof Johansson, 2011-03-30 03:39:54]

Adding Mandeep and Anush since they tend to keep an eye on the ebuild side of
things.

Note that we should be able to build with both fbconsole and working X server
within the not too distant future, so we should be able to revert this then.

[Mandeep Singh Baines, 2011-04-01 15:04:51]

What if we just have it always on? What are the downsides: effect on image size,
boot time? Would like to keep optional stuff to a minimum.

[Olof Johansson, 2011-04-04 17:40:32]

The current video driver for tegra doesn't allow more than one client (and
framebuffer counts as one). So you can't have framebuffer console enabled and
then start X.

This is being fixed, so when that happens we can back out this change.

[Tom Wai-Hong Tam, 2011-04-06 02:03:58]

Olof are right. In the base ChromeOS image, the framebuffer console is turned
off in order to start X. But in recovery and factory shims, we need console
output but not X. So this fbconsole USE flag can do this customization.

[Tom Wai-Hong Tam, 2011-04-25 08:54:09]

Hi, could you kindly give me an approval if these ebuilds look good?

Thanks.

[Mandeep Singh Baines, 2011-04-25 14:35:20]

LGTM