mpc83xx: Add NAND boot support for MPC8315E-RDB boards

The core support for NAND booting is there already, so this patch
is pretty straightforward.

There is one trick though: top level Makefile expects nand_spl to
be in nand_spl/board/$(BOARDDIR), but we can fully reuse the code
from mpc8313erdb boards, and so to not duplicate the code we just
symlink nand_spl/board/freescale/mpc8315erdb to mpc8313erdb.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>

o silence make during ln echo
o update documentation
o and avoid:

$ ./MAKEALL MPC8315ERDB_NAND
Configuring for MPC8315ERDB board...
sdram.o: In function `fixed_sdram':
/home/r1aaha/git/u-boot/nand_spl/board/freescale/mpc8313erdb/sdram.c:72: undefined reference to `udelay'

by renaming udelay -> __udelay in the spirit of commit
3eb90bad651fab39cffba750ec4421a9c01d60e7 "Generic udelay() with watchdog
support".

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
diff --git a/MAKEALL b/MAKEALL
index ab1bb6f..1b78778 100755
--- a/MAKEALL
+++ b/MAKEALL
@@ -362,6 +362,7 @@
 	MPC8313ERDB_33	\
 	MPC8313ERDB_NAND_66	\
 	MPC8315ERDB	\
+	MPC8315ERDB_NAND	\
 	MPC8323ERDB	\
 	MPC832XEMDS	\
 	MPC832XEMDS_ATM	\