payloads: Reorganize Kconfig for external payloads

The integration of external payloads in coreboot
is a bit messy. You have to change the to level Kconfig
file for every payload (something that we recently fixed
for mainboards and chipsets). This means that updating
e.g. the SeaBIOS version requires a change outside of the
SeaBIOS directory.

With this patch you can create a new directory under
payloads/external and place a Kconfig and Kconfig.name
file in there, and the payload will automatically show
up when you do "make menuconfig".

Change-Id: I293abcb8eae581d4b3934e64897c0d339a27e7c1
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10828
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

payloads/external/GRUB2/Kconfig

@@ -0,0 +1,17 @@
+if PAYLOAD_GRUB2
+
+choice
+	prompt "GRUB2 version"
+	default GRUB2_MASTER
+
+config GRUB2_MASTER
+	bool "HEAD"
+	help
+	  Newest GRUB2 version
+
+endchoice
+
+config PAYLOAD_FILE
+	default "payloads/external/GRUB2/grub2/build/default_payload.elf"
+
+endif

payloads/external/GRUB2/Kconfig.name

@@ -0,0 +1,8 @@
+config PAYLOAD_GRUB2
+	bool "GRUB2"
+	help
+	  Select this option if you want to build a coreboot image
+	  with a GRUB2 payload. If you don't know what this is
+	  about, just leave it enabled.
+
+	  See http://coreboot.org/Payloads for more information.