fw_env: add NAND support

Add support for environment in NAND with automatic NOR / NAND recognition,
including unaligned environment, bad-block skipping, redundant environment
copy.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>

tools/env/README

@@ -22,9 +22,11 @@ following lines are relevant:
 #define DEVICE1_OFFSET    0x0000
 #define ENV1_SIZE         0x4000
 #define DEVICE1_ESIZE     0x4000
+#define DEVICE1_ENVSECTORS     2
 #define DEVICE2_OFFSET    0x0000
 #define ENV2_SIZE         0x4000
 #define DEVICE2_ESIZE     0x4000
+#define DEVICE2_ENVSECTORS     2
 
 Current configuration matches the environment layout of the TRAB
 board.
@@ -46,3 +48,7 @@ then 1 sector.
 
 DEVICEx_ESIZE defines the size of the first sector in the flash
 partition where the environment resides.
+
+DEVICEx_ENVSECTORS defines the number of sectors that may be used for
+this environment instance. On NAND this is used to limit the range
+within which bad blocks are skipped, on NOR it is not used.

tools/env/fw_env.config

 # Configuration file for fw_(printenv/saveenv) utility.
 # Up to two entries are valid, in this case the redundand
 # environment sector is assumed present.
+# Notice, that the "Number of sectors" is ignored on NOR.
 
-# MTD device name	Device offset	Env. size	Flash sector size
+# MTD device name	Device offset	Env. size	Flash sector size	Number of sectors
 /dev/mtd1		0x0000		0x4000		0x4000
 /dev/mtd2		0x0000		0x4000		0x4000
+
+# NAND example
+#/dev/mtd0		0x4000		0x4000		0x20000			2