NAND: Do not write or read a whole block if it is larger than the environment

Environment can be smaller than NAND block size, do not need to read a whole
block and minimum for writing is one page. Also remove an unused variable.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>