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/libc/console.c

@@ -76,6 +76,9 @@ void console_add_input_driver(struct console_input_driver *in)
 	/* Check if this driver was already added to the console list */
 	if (input_driver_exists(in))
 		return;
+	/* Flush out the driver input buffer. */
+	while (in->havekey())
+		in->getchar();
 	in->next = console_in;
 	console_in = in;
 }