video/console: automatically select a font

The frame buffer console needs at least one font to be built into
the kernel, so add the necessary Kconfig magic to guarantee that
one of the available font is always on. If a user accidentally
disables all fonts manually, the 8x16 font will be selected
anyway.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>