selftests/nolibc: determine $(srctree) first

selftests/nolibc: determine $(srctree) first

The nolibc-test Makefile includes various other Makefiles from the tree.
At first these are included with relative paths like
"../../../build/Build.include" but as soon as $(srctree) is set up,
the inclusions use that instead to build full paths.

To keep the style of inclusions consistent, perform the setup
$(srctree) as early as possible and use it for all inclusions.

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-9-c20f2f5fc7c2@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>