payloads: Update GRUB stable from 2.02 to 2.04

GRUB 2.04 was released on July 5th, 2019, so update. The only change-log
is the git history. Some coreboot related changes are listed below.

1.  coreboot: Changed cbmemc to support updated console format from
    coreboot.
2.  ahci: Increase time-out from 10 s to 32 s
3.  normal/menu: Do not treat error values as key presses

When building from the git repository, `./bootstrap.sh` needs to be run
to set up Gnulib. Ignore the exit code, as older versions might not have
this script.

Change-Id: Iab0b87164ed86f15d3415af935998b59e0d76c45
Signed-off-by: Pablo <42.pablo.ms@gmail.com>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34104
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

payloads/external/GRUB2/Kconfig

@@ -5,7 +5,7 @@ choice
 	default GRUB2_STABLE
 
 config GRUB2_STABLE
-	bool "2.02"
+	bool "2.04"
 	help
 	  Stable GRUB2 version