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>

Documentation/mainboard/emulation/qemu-power9.md

@@ -1,7 +1,19 @@
 # QEMU PPC64 emulator
 This page describes how to build and run coreboot for QEMU/PPC64.
-You can configure ELF payload via `make menuconfig` (make sure "ROM chip size"
-is large enough).
+
+## Building coreboot
+```bash
+make defconfig KBUILD_DEFCONFIG=configs/config.emulation_qemu_power9
+make
+```
+
+This builds coreboot with no payload.
+
+## Payloads
+You can configure ELF or `skiboot` payload via `make menuconfig`. In either case
+you might need to adjust "ROM chip size" and make it large enough to accommodate
+the payload (see how much space it needs in the error you get if it doesn't
+fit).
 
 ## Running coreboot in QEMU
 ```bash
@@ -15,16 +27,26 @@ qemu-system-ppc64 -M powernv,hb-mode=on \
 
 - The default CPU in QEMU for AArch64 is a 604. You specify a suitable
 PowerPC CPU via `-cpu power9`.
-- By default Hostboot mode is off and it needs to be turned on to run coreboot
+- By default Hostboot mode is off and needs to be turned on to run coreboot
 as a firmware rather than like an OS.
 - `-bios` specifies initial program (bootloader should suffice, but whole image
 works fine too).
 - `-drive` specifies image for emulated flash device.
 
-## Building coreboot
+## Running with a kernel
+Loading `skiboot` (built automatically by coreboot or otherwise) allows
+specifying kernel and root file system to be run.
+
 ```bash
-make defconfig KBUILD_DEFCONFIG=configs/config.emulation_qemu_power9
-make
+qemu-system-ppc64 -M powernv,hb-mode=on \
+                  -cpu power9 \
+                  -bios build/coreboot.rom \
+                  -drive file=build/coreboot.rom,if=mtd \
+                  -serial stdio \
+                  -display none \
+                  -kernel zImage \
+                  -initrd initrd.cpio.xz
 ```
 
-This builds coreboot with no payload.
+- Specify path to your kernel via `-kernel`.
+- Specify path to your rootfs via `-initrd`.