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>

payloads/external/skiboot/Makefile

@@ -0,0 +1,36 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
+build_dir=$(CURDIR)/build
+skiboot_dir=$(CURDIR)/skiboot
+skiboot_git_repo=$(CONFIG_SKIBOOT_GIT_REPO)
+skiboot_revision=$(CONFIG_SKIBOOT_REVISION)
+skiboot_elf=$(build_dir)/skiboot.elf
+skiboot_cross=$(or $(CROSS),powerpc64-linux-gnu-)
+
+unexport $(COREBOOT_EXPORTS)
+
+.PHONY: all clean distclean
+
+all: $(skiboot_elf)
+
+$(skiboot_elf): | $(skiboot_dir) $(build_dir)
+	+$(MAKE) -C $(skiboot_dir) CROSS="$(skiboot_cross)"
+	cp $(skiboot_dir)/skiboot.elf $@
+	# skiboot is always built with debug information due to unconditional -ggdb
+	$(skiboot_cross)strip $@
+
+$(skiboot_dir):
+	git clone $(skiboot_git_repo) $(skiboot_dir)
+	git -C $(skiboot_dir) checkout $(skiboot_revision)
+
+$(build_dir):
+	mkdir -p $(build_dir)
+
+distclean: clean
+	rm -rf $(skiboot_dir)
+
+clean:
+	# Redefine RM because it's used like `$(RM) non-existent-file`
+	# Also ignore useless messages about removing test files
+	[ ! -d $(skiboot_dir) ] || $(MAKE) -C $(skiboot_dir) RM="rm -rf" clean > /dev/null
+	rm -rf $(build_dir)