loader.mk: clean md.o even if MD_IMAGE_SIZE not defined

We don't normally provide special handling for optionally-included src
files, but md.o depends on both md.c and the value of ${MD_IMAGE_SIZE}.
Previously if one built with MD_IMAGE_SIZE, executed "make clean", and
then built with a different MD_IMAGE_SIZE md.o would not be rebuilt.

Reported by: Zakary Nafziger
Sponsored by: The FreeBSD Foundation