Add support for dataflash to U-boot environment settings tool.

* The sector size for SPI-dataflash (like AT45 flashes) are not always
  a power-of-2. So, the sector calculations are rewritten such that it
  works for either power-of-2 as any size sectors.
* Make the flash sector size optional in case it is the same value as
  the environment size.

Signed-off-by: Remy Bohmer <linux@bohmer.net>
diff --git a/tools/env/fw_env.config b/tools/env/fw_env.config
index c8f12cf..8e21d5a 100644
--- a/tools/env/fw_env.config
+++ b/tools/env/fw_env.config
@@ -1,11 +1,19 @@
 # Configuration file for fw_(printenv/saveenv) utility.
 # Up to two entries are valid, in this case the redundant
 # environment sector is assumed present.
-# Notice, that the "Number of sectors" is ignored on NOR.
+# Notice, that the "Number of sectors" is ignored on NOR and SPI-dataflash.
+# Futhermore, if the Flash sector size is ommitted, this value is assumed to
+# be the same as the Environment size, which is valid for NOR and SPI-dataflash
 
+# NOR example
 # MTD device name	Device offset	Env. size	Flash sector size	Number of sectors
 /dev/mtd1		0x0000		0x4000		0x4000
 /dev/mtd2		0x0000		0x4000		0x4000
 
+# MTD SPI-dataflash example
+# MTD device name	Device offset	Env. size	Flash sector size	Number of sectors
+#/dev/mtd5		0x4200		0x4200
+#/dev/mtd6		0x4200		0x4200
+
 # NAND example
 #/dev/mtd0		0x4000		0x4000		0x20000			2