src: change coreboot to lowercase

The word 'coreboot' should always be written in lowercase, even at the
start of a sentence.

Change-Id: I7945ddb988262e7483da4e623cedf972380e65a2
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20029
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/drivers/aspeed/common/ast_dp501.c

@@ -2,7 +2,7 @@
  * This file is part of the coreboot project.
  *
  * File taken from the Linux ast driver (v3.18.5)
- * Coreboot-specific includes added at top and/or contents modified
+ * coreboot-specific includes added at top and/or contents modified
  * as needed to function within the coreboot environment.
  *
  * This program is free software; you can redistribute it and/or modify