boards.cfg: Delete the equivalent entries

There are some entries which produce the same binaries:
 - ep8248E           is equivalent to ep8248
 - MPC8360ERDK_66    is equivalent to MPC8360ERDK
 - Adder87x/AdderUSB is equivalent to Adder
 - EVB64260_750CX    is equivalent to EVB64260

I also notice
 - Lite5200           is equivalent to icecube_5200
 - Lite5200_LOWBOOT   is equivalent to icecube_5200_LOWBOOT
 - Lite5200_LOWBOOT08 is equivalent to icecube_5200_LOWBOOT08
But I am keeping them.
(Wolfgang suggested to do so because Lite5200* are referenced
in misc documents.)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>