payloads/external: add skiboot (for QEMU/Power9)

Add an option to build skiboot as a payload. This makes QEMU Power9
board simpler to use as skiboot is necessary anyway.

Change-Id: I0b49ea7464c97cc2ff0d5030629deed549851372
Signed-off-by: Igor Bagnucki <igor.bagnucki@3mdeb.com>
Signed-off-by: Krystian Hebel <krystian.hebel@3mdeb.com>
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58656
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
diff --git a/payloads/Makefile.inc b/payloads/Makefile.inc
index 83194c2..a182bd6 100644
--- a/payloads/Makefile.inc
+++ b/payloads/Makefile.inc
@@ -28,6 +28,7 @@
 payloads/external/GRUB2 \
 payloads/external/LinuxBoot \
 payloads/external/Yabits \
+payloads/external/skiboot \
 
 force-payload: