Fix Makefile install targets

Fix Makefile install targets

Some Makefiles were still referring to libutil_com.a, even though we
stopped producing that file with commit 74518b8b691c ("elflink: Make
ELF the default object format"). Delete all references to avoid the
following install error,

	install: cannot stat `libutil_com.a': No such file or directory

Remove the install target from com32/elflink as we no longer build any
of the test modules, and so don't need to install them.

Also, we need to add ldlinux.c32 to the list of modules that *must* be
installed for all targets, since it is required for Syslinux to
function correctly.

Reported-by: Dag Wieers <dag@wieers.com>
Reported-by: Michael D. Setzer II <mikes@kuentos.guam.net>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>