elflink: ldlinux should not have any dependencies

elflink: ldlinux should not have any dependencies

ldlinux currently requires the get_key() symbol, which means it has a
dependency on libutil_com.c32. ldlinux.c32 really should be a
standalone module so let's move get_key.c into ldlinux/.

However, moving get_key.c creates a new problem - native linux
applications such as com32/samples/keytest.c require the get_key()
symbol which is currently exported by libutil_lnx.a. To fix this, we
create a new native linux library that exports any required symbols
from ldlinux. With this change we need to update com32/Makefile so
that we build com32/elfink/ldlinux before com32/samples, and seeing as
ldlinux now has no dependencies, we may as well move it to the front
of $SUBDIRS.

Also, update elf_gen_dep.sh to skip ldlinux.c32 and not search for
dependencies for any of its unresolved symbols. This way, if anyone
inadvertently introduces a dependency that cannot be resolved by the
core, ldlinux.c32 will fail to load at runtime (which guarantees that
the newly introduced dependency won't go unnoticed).

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>