payloads/external/SeaBIOS: Upgrade stable from 1.7.2.1 to 1.7.4

SeaBIOS 1.7.4 was released in December 2013 [1] and, besides other
things, supports writing debug messages to CBMEM console.

The new SeaBIOS Kconfig option `DEBUG_COREBOOT` has to be added to the
SeaBIOS configuration file `.config` as otherwise the SeaBIOS build
from within coreboot (`PAYLOAD_SEABIOS`) is interrupted as it is
detected as a new option.

This option was already added and enabled in commit 7c1a49bc [1]

	SeaBIOS: have coreboot pass the choice to run optionroms in parallel

so SeaBIOS messages are now written to the CBMEM console.

Successfully tested on the Asus M2V-MX SE.

[1] http://seabios.org/Releases
[2] http://review.coreboot.org/5443

Change-Id: I675a50532735b4921a664e4b24d98be17b9a1002
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/5093
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

payloads/external/SeaBIOS/Makefile.inc

@@ -1,5 +1,5 @@
 TAG-$(CONFIG_SEABIOS_MASTER)=origin/master
-TAG-$(CONFIG_SEABIOS_STABLE)=88cb66ea542906ffff8a80ef397b9e3adbb33116
+TAG-$(CONFIG_SEABIOS_STABLE)=96917a8ed761f017fc8c72ba3b9181fbac03ac59
 
 unexport KCONFIG_AUTOHEADER
 unexport KCONFIG_AUTOCONFIG

src/Kconfig

@@ -599,7 +599,7 @@ choice
 	depends on PAYLOAD_SEABIOS
 
 config SEABIOS_STABLE
-	bool "1.7.2.1"
+	bool "1.7.4"
 	help
 	  Stable SeaBIOS version
 config SEABIOS_MASTER