video console: allow font size configuration at runtime

Allow font size configuration at runtime for console_simple.c
driver. This needed for unit testing different fonts.

Configuring is done by `font` command, also used for font
selection in true type console.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>