mpc83xx: fix out-of-tree mpc8315 nand build

commit 2e95004d "mpc83xx: Add NAND boot support
for MPC8315E-RDB boards" symlinked nand_spl/board/freescale/mpc8315erdb to
mpc8313erdb in order to not duplicate code.

Since the main makefile builds nand_spl/board/$(BOARDDIR) (which makes sense),
and the board Makefile and linker script are the only two necessary files
to enable out-of-tree building, and other boards have duplicated nand makefiles
(e.g. 8536ds & 8569mds), it only seems prudent to copy these two files in the
name of the 8315 too.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Cc: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Wolfgang Denk <wd@denx.de>