We have a board with several revisions. The older ones use a nor flash
with 64k erase size, while the newer have a flash with 4k sectors. The
environment size is 8k.

Currently, we have to put a column containing 0x10000 (64k) in
fw_env.config in order for it to work on the older boards. But that
ends up wasting quite a lot of time on the newer boards that could
just erase the 8k occupied by the environment - strace says the 64k
erase takes 0.405 seconds. With this patch, as expected, that's about
an 8-fold better, at 0.043 seconds.

Having different fw_env.config files for the different revisions is
highly impractical, and the correct information is already available
right at our fingertips. So use the erasesize returned by the
MEMGETINFO ioctl when the fourth and fifth columns (sector size and
#sectors, respectively) are absent or contain 0, a case where the
logic previously used to use the environment size as erase size (and
consequently computed ENVSECTORS(dev) as 1).

As I'm only testing this on a NOR flash, I'm only changing the logic
for that case, though I think it should be possible for the other
types as well.
Signed-off-by: NRasmus Villemoes <rasmus.villemoes@prevas.dk>