Introduce a new VT mode KD_TRANSPARENT which endevours to leave the current
content of the framebuffer untouched.  This allows the bootloader to insert
a graphical splash and have the kernel maintain it until the OS splash
can take over.  When we finally switch away (either through programs like
plymouth or manually) the content is lost and the VT reverts to text mode.

BugLink: http://bugs.launchpad.net/bugs/1308685Signed-off-by: Andy Whitcroft <apw@canonical.com>