external/LinuxBoot: Fix cleanup mechanism

`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>

payloads/external/LinuxBoot/Makefile

@@ -1,7 +1,7 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
 project_dir=linuxboot
-kernel_dir=$(project_dir)/kernel
+kernel_names=$(shell ls $(project_dir) | grep kernel)
 
 unexport $(COREBOOT_EXPORTS)
 
@@ -52,9 +52,12 @@ endif
 linuxboot: kernel initramfs_compressed
 
 clean:
-	if [ -d "$(kernel_dir)" ]; then rm -rf $(kernel_dir); fi
+	if [ ! -z "$(kernel_names)" ]; then \
+	       	rm -rf $(project_dir)/kernel*; \
+	fi
 	rm -f $(project_dir)/u-root
 	rm -f $(project_dir)/initramfs*
+	rm -f $(project_dir)/bzImage
 
 distclean:
 	rm -rf $(project_dir)