selftests/nolibc: don't embed initramfs into kernel image

selftests/nolibc: don't embed initramfs into kernel image

When the initramfs is embedded into the kernel each rebuild of it will
trigger a full kernel relink and all the expensive postprocessing steps.

Currently nolibc-test and therefore the initramfs are always rebuild,
even without source changes, leading to lots of slow kernel relinks.

Instead of linking the initramfs into the kernel assemble it manually
and pass it explicitly to qemu.
This avoids all of the kernel relinks.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Link: https://lore.kernel.org/r/20230917-nolibc-initramfs-v2-1-f0f293a8b198@weissschuh.net