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/Makefile b/Makefile
index ed6156f..7dca112 100644
--- a/Makefile
+++ b/Makefile
@@ -2261,8 +2261,12 @@
 		echo "CONFIG_NAND_U_BOOT = y" >> $(obj)include/config.mk ; \
 	fi ;
 
+MPC8315ERDB_NAND_config \
 MPC8315ERDB_config: unconfig
-	@$(MKCONFIG) -a MPC8315ERDB ppc mpc83xx mpc8315erdb freescale
+	@if [ "$(findstring _NAND_,$@)" ] ; then \
+		ln -sf mpc8313erdb nand_spl/board/freescale/mpc8315erdb ; \
+	fi ;
+	@$(MKCONFIG) -t $(@:_config=) MPC8315ERDB ppc mpc83xx mpc8315erdb freescale
 
 MPC8323ERDB_config:	unconfig
 	@$(MKCONFIG) -a MPC8323ERDB ppc mpc83xx mpc8323erdb freescale