fw_env: autodetect NAND erase size and env sectors

As already done for NOR chips, if device ESIZE and ENVSECTORS static
configurations are both zero, then autodetect them at runtime.

Cc: Joe Hershberger <joe.hershberger@ni.com>
cc: Stefan Agner <stefan@agner.ch>
cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Anthony Loiseau <anthony.loiseau@allcircuits.com>