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
3eb90bad65 "Generic udelay() with watchdog
support".

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>

doc/README.mpc8315erdb

@@ -15,6 +15,18 @@ Freescale MPC8315ERDB Board
 	  4321		  4321
 	(where the '*' indicates the position of the tab of the switch.)
 
+	To boot the image at the beginning of NAND flash, use these
+	DIP switch settings for S3 S4:
+
+	+------+	+------+
+	| *    |	|  *** |
+	|  *** |	| *    |
+	+------+ ON	+------+ ON
+	  4321		  4321
+	(where the '*' indicates the position of the tab of the switch.)
+
+	When booting from NAND, use u-boot-nand.bin, not u-boot.bin.
+
 2.	Memory Map
 	The memory map looks like this:
 
@@ -26,6 +38,9 @@ Freescale MPC8315ERDB Board
 	0xe060_0000	0xe060_7fff	NAND FLASH (CS1) 32K
 	0xfe00_0000	0xfe7f_ffff	NOR FLASH (CS0)	 8M
 
+	When booting from NAND, NAND flash is CS0 and NOR flash
+	is CS1.
+
 3.	Definitions
 
 3.1	Explanation of NEW definitions in:
@@ -43,13 +58,15 @@ Freescale MPC8315ERDB Board
 
 	export CROSS_COMPILE=your-cross-compiler-prefix-
 	make distclean
-	make MPC8315ERDB_config
+	make MPC8315ERDB_config (or MPC8315ERDB_NAND_config for u-boot-nand.bin)
 	make all
 
 5.	Downloading and Flashing Images
 
 5.1	Reflash U-boot Image using U-boot
 
+	NOR flash:
+
 	tftp 40000 u-boot.bin
 	protect off all
 	erase fe000000 fe1fffff
@@ -60,6 +77,15 @@ Freescale MPC8315ERDB Board
 	You have to supply the correct byte count with 'xxxx'
 	from the TFTP result log.
 
+	NAND flash:
+
+	=>tftpboot $loadaddr <filename>
+	=>nand erase 0 0x80000
+	=>nand write $loadaddr 0 0x80000
+
+	...where 0x80000 is the filesize rounded up to
+	the next 0x20000 increment.
+
 5.2	Downloading and Booting Linux Kernel
 
 	Ensure that all networking-related environment variables are set
@@ -76,5 +102,4 @@ Freescale MPC8315ERDB Board
 
 6	Notes
 
-	Booting from NAND flash is not yet supported.
 	The console baudrate for MPC8315ERDB is 115200bps.