Remove board support for afeb9260, tny_a9260, and sbc35_a9g20.
They have not been converted into Generic Board yet.
See doc/README.generic-board for details.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Sergey Lapin <slapin@ossfans.org>
Cc: Albin Tonnerre <albin.tonnerre@free-electrons.com>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>