libpayload/libc/console: Flush input driver buffer on init

When console input driver registers itself, perform flush of input
buffer to avoid interpreting any stale key presses before libpayload
is run.

keyboard.c: Remove the redundant buffer flush.
8250.c: Ensure that serial_hardware_is_present is set before call to
add input driver.

BUG=b:37273808
TEST=Verified that any key presses in serial console before payload is
up do not have any effect after the payload starts running.

Change-Id: I46f1b6715ccf6418f5b2c741bf90db2ece26a60d
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19345
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

payloads/libpayload/drivers/keyboard.c

@@ -310,9 +310,6 @@ void keyboard_init(void)
 	if (inb(0x64) == 0xFF)
 		return;
 
-	/* Empty keyboard buffer */
-	while (keyboard_havechar()) keyboard_getchar();
-
 	console_add_input_driver(&cons);
 }