Out of tree builds are much more convenient when building for multiple architectures or configurations in parallel. Only absolute O= parameters are supported as Makefile.include will always resolve relative paths in relation to $(srctree) instead of the current directory. Add a call to "make outputmakefile" to verify that the sourcetree is clean. This is based on Zhangjins out-of-tree patch. It extends that work for get_init_cpio support and also drops relative O= specifications explicitly. Link: https://lore.kernel.org/lkml/06d96bd81fe812a9718098a383678ad3beba98b1.1691215074.git.falcon@tinylab.org/ Co-developed-by: Zhangjin Wu <falcon@tinylab.org> Signed-off-by: Zhangjin Wu <falcon@tinylab.org> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net> Acked-by: Willy Tarreau <w@1wt.eu> Link: https://lore.kernel.org/r/20231031-nolibc-out-of-tree-v1-3-47c92f73590a@weissschuh.net