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: f86ca5ad ("Introduce Verifying Program Loader (VPL)")
Signed-off-by: Tobias Deiminger <tdmg@linutronix.de>