sbc8641d: set proper environment sector size.

When debugging an env fail due to too small a malloc pool, it
was noted that the env write was 256k.  But the device sector
size is 1/2 that, as can be seen from "fli" output:

Bank # 1: CFI conformant flash (16 x 16)  Size: 16 MB in 131 Sectors
  Intel Extended command set, Manufacturer ID: 0x89, Device ID: 0x1888
  Erase timeout: 4096 ms, write timeout: 1 ms
  Buffer write timeout: 2 ms, buffer size: 64 bytes

  Sector Start Addresses:
  FF000000 E RO   FF020000 E RO   FF040000 E RO   FF060000 E RO   FF080000 E RO
  FF0A0000 E RO   FF0C0000 E RO   FF0E0000 E RO   FF100000 E RO   FF120000 E RO
  [...]
  FFF00000   RO   FFF20000   RO   FFF40000   RO   FFF60000   RO   FFF80000   RO
  FFFA0000   RO   FFFC0000   RO   FFFE0000 E RO   FFFE8000   RO   FFFF0000 E RO
  FFFF8000   RO
=>

The desired env sector is FFF40000->FFF60000, or 0x20000 in length,
just after the 256k u-boot image which starts @ FFF00000.
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

include/configs/sbc8641d.h

@@ -473,7 +473,7 @@
  */
 #define CONFIG_ENV_IS_IN_FLASH	1
 #define CONFIG_ENV_ADDR		(CONFIG_SYS_MONITOR_BASE + 0x40000)
-#define CONFIG_ENV_SECT_SIZE	0x40000	/* 256K(one sector) for env */
+#define CONFIG_ENV_SECT_SIZE	0x20000	/* 128k(one sector) for env */
 #define CONFIG_ENV_SIZE		0x2000
 
 #define CONFIG_LOADS_ECHO	1	/* echo on for serial download */