Blackfin: unify u-boot linker scripts

All the Blackfin linker scripts were duplicated across the board dirs with
no difference save from the semi-often used ENV_IS_EMBEDDED option.  So
unify all of them in the lib_blackfin/ dir and for the few boards that
need to embedded the environment directly, add a LDS_BOARD_TEXT define for
them to customize via their board config file.  This is much simpler than
forcing them to duplicate the rest of the linker script.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>