Expose the high bit of the VGA text attribute byte via the ANSI SGR
parameters 5 ("blink on") and 25 ("blink off").

Note that some video cards (and virtual machines) may display a high
intensity background colour instead of blinking text.

Signed-off-by: Christian Nilsson <nikize@gmail.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>