vgabios: Support emulating text mode attributes while in graphics mode

Add support for simple text mode attribute emulation while in graphics
mode.  This seems to help with text highlighting and colors with a few
boot-loaders.  Turn it enabled only for CBVGA vgabios.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
vgabios: Add software cursor capability

Add mechanism for drawing a cursor to the framebuffer to implement a
cursor in software.  The timer interrupt is "hooked" so that the
cursor can blink.  This can be useful for "coreboot native vga".

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>