framebuffer: fix border color

The framebuffer code uses the current background color to fill the border
when switching consoles, however, this results in inconsistent behavior.
For example:
- start Midnigh Commander
- the border is black
- switch to another console and switch back
- the border is cyan
- type something into the command line in mc
- the border is cyan
- switch to another console and switch back
- the border is black
- press F9 to go to menu
- the border is black
- switch to another console and switch back
- the border is dark blue

When switching to a console with Midnight Commander, the border is random
color that was left selected by the slang subsystem.

This patch fixes this inconsistency by always using black as the
background color when switching consoles.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>