wandboard: move environment partition farther from u-boot.img

Recently I started to notice that u-boot.img built for Wandboard
by some toolchains becomes so large that it basically overlaps with
U-Boot environment area on SD-card.

According to
http://wiki.wandboard.org/index.php/Boot-process#sdcard_boot_data_layout
Wandboard's SD-card layout is as follows:
------------------------------>8---------------------------
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Fabio Estevam <fabio.estevam@nxp.com>

==========================================================
1. 0x00000000           Reserved For MBR
2. 0x00000200   512     Secondary Image Table (optional)
3. 0x00000400   1024    uBoot Image (Starting From IVT)
4. 0x00060000   393216  start of uboot env (size:8k)
5. 0x00062000           end of uboot env
6. 0x00100000   1048576 Linux kernel start
7. 0x0076AC00   7777280 start of partition 1
------------------------------>8---------------------------

So for U-Boot we have 383kB (392192 bytes).

But in up to date U-Boot for Wandboard we build separately
 a) SPL
 b) u-boot.img

which gives us a bit more detailed SD-card layout:
------------------------------>8---------------------------
==========================================================
1. 0x00000000           Reserved For MBR
2. 0x00000200   512     Secondary Image Table (optional)
3. 0x00000400   1024    SPL
4. 0x00011400   70656   u-boot.img
5. 0x00060000   393216  start of uboot env (size:8k)
6. 0x00062000           end of uboot env
...
------------------------------>8---------------------------

>From that layout we may calculate amount of space reserved for
u-boot.img. It's just 315kb (322560 bytes).

Now if I build U-Boot with Sourcery CodeBench ARM 2014.05 produced
u-boot.img is already more than we expected
(323840 bytes instead of "< 322560"):
------------------------------>8---------------------------
ls -la u-boot.img
-rw-rw-r-- 1 user user 323840 Jul  5 07:38 u-boot.img
------------------------------>8---------------------------

Funny enough if I rebuild U-Boot with ARM toolchain available in
my Fedora 23 distro u-boot.img becomes a little bit smaller:
------------------------------>8---------------------------
ls -la u-boot.img
-rw-rw-r-- 1 user user 322216 Jul  5 07:39 u-boot.img
------------------------------>8---------------------------

What's worse this problem might not affect people most of the time
because what happens people would just copy u-boot.img on SD-card and
live in happiness with it... well until somebody attempts to save
environment in U-Boot with "saveenv" command which will simply
overwrite the very end of u-boot.img.
That will lead to unusable SD-card until user dd u-boot.img on
SD-card again.

I may foresee this issue in the future to become more visible once we
add more features in U-Boot for Wandboard or just existing code base
becomes bulkier and people will consistently get larger u-boot.img
files produced.

IMHO there's an obvious solution for all that - just move U-Boot's env
to the very end of the gap between U-Boot and the first real partition
on the SD-card. This patch will follow
8fb9eea565 ("mx6sabre_common: Fix U-Boot corruption after 'saveenv'").
So env is still not in the very end of the gap (obviously 256kb is way
too much for U-Boot's env) but at least we have now the same
partitioning for i.MX6 boards.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Peter Robinson <pbrobinson@gmail.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Wolfgang Denk <wd@denx.de>

include/configs/wandboard.h

@@ -193,7 +193,7 @@
 #define CONFIG_ENV_SIZE			(8 * 1024)
 
 #define CONFIG_ENV_IS_IN_MMC
-#define CONFIG_ENV_OFFSET		(6 * 64 * 1024)
+#define CONFIG_ENV_OFFSET		(768 * 1024)
 #define CONFIG_SYS_MMC_ENV_DEV		0
 
 #endif			       /* __CONFIG_H * */