[bios] Add ANSI blink attribute

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>

src/arch/i386/firmware/pcbios/bios_console.c

 #define ATTR_FCOL_YELLOW	0x06
 #define ATTR_FCOL_WHITE		0x07
 
+#define ATTR_BLINK		0x80
+
 #define ATTR_BCOL_MASK		0x70
 #define ATTR_BCOL_BLACK		0x00
 #define ATTR_BCOL_BLUE		0x10
 			bios_attr = ATTR_DEFAULT;
 		} else if ( aspect == 1 ) {
 			bios_attr |= ATTR_BOLD;
+		} else if ( aspect == 5 ) {
+			bios_attr |= ATTR_BLINK;
 		} else if ( aspect == 22 ) {
 			bios_attr &= ~ATTR_BOLD;
+		} else if ( aspect == 25 ) {
+			bios_attr &= ~ATTR_BLINK;
 		} else if ( ( aspect >= 30 ) && ( aspect <= 39 ) ) {
 			bios_attr &= ~ATTR_FCOL_MASK;
 			bios_attr |= bios_attr_fcols[ aspect - 30 ];