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>