payloads/LinuxBoot: Add uImage to clean target

uImages are generated for non-x86 arch.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: Icb1184497087d66a7cc6fd27402365a028cc4eaf
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78643
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>
diff --git a/payloads/external/LinuxBoot/Makefile b/payloads/external/LinuxBoot/Makefile
index 29339f14..87066c2 100644
--- a/payloads/external/LinuxBoot/Makefile
+++ b/payloads/external/LinuxBoot/Makefile
@@ -54,6 +54,7 @@
 	rm -f  build/u-root
 	rm -f  build/initramfs*
 	rm -f  build/bzImage
+	rm -f  build/uImage
 
 distclean:
 	rm -rf build