mkelfimage: remove

It's not been needed for years, is definitely not needed now
that cbfstool parses bzImages, and its presence keeps confusing
people.

Also, rewrite history. We never mentioned mkelfimage in the
documentation. Never, ever, ever.

Change-Id: Id96a57906ba6a423b06a8f4140d2efde6f280d55
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/7021
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <gaumless@gmail.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>