`make clean` never thoroughly removed LinuxBoot build artifacts. This change checks for kernel directories present in the project directory and deletes all of them, if found. Signed-off-by: Patrik Tesarik <patrik.tesarik@9elements.com> Change-Id: Ia056ac6608e3631dfc270ba5c2f32216c3e1ac50 Reviewed-on: https://review.coreboot.org/c/coreboot/+/65098 Reviewed-by: Arthur Heymans <arthur@aheymans.xyz> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>