Commit 7d36a1b ("[build] Explicitly link efilink against -liberty")
introduced a dependency on libiberty to cope with old versions of
libbfd.  This commit dates from 2008 and seems to apply only to what
are now extremely old versions of libbfd (prior to binutils 2.12).

There are systems (such as current Debian) which do not include
libiberty within the binutils packages.  On such systems, our build
dependency on libiberty represents a pointless hurdle.

Remove the explicit dependency on libiberty, hoping that there are no
modern systems where this will cause a problem.

Suggested-by: Ben Hildred <42656e@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>