Kbuild: Fix cleanup of VPL

VPL artifacts like example vpl/u-boot-vpl are currently not removed by
'make clean'.

We can clean them just as it's already done for SPL and TPL.

Fixes: f86ca5ad8f78 ("Introduce Verifying Program Loader (VPL)")
Signed-off-by: Tobias Deiminger <tdmg@linutronix.de>
diff --git a/Makefile b/Makefile
index 813ad3e..8d9aef4 100644
--- a/Makefile
+++ b/Makefile
@@ -2152,7 +2152,7 @@
 
 # Directories & files removed with 'make clean'
 CLEAN_DIRS  += $(MODVERDIR) \
-	       $(foreach d, spl tpl, $(patsubst %,$d/%, \
+	       $(foreach d, spl tpl vpl, $(patsubst %,$d/%, \
 			$(filter-out include, $(shell ls -1 $d 2>/dev/null))))
 
 CLEAN_FILES += include/bmp_logo.h include/bmp_logo_data.h \
@@ -2167,7 +2167,7 @@
 	       idbloader-spi.img lib/efi_loader/helloworld_efi.S
 
 # Directories & files removed with 'make mrproper'
-MRPROPER_DIRS  += include/config include/generated spl tpl \
+MRPROPER_DIRS  += include/config include/generated spl tpl vpl \
 		  .tmp_objdiff doc/output include/asm
 
 # Remove include/asm symlink created by U-Boot before v2014.01