fw_env: calculate default number of env sectors
The assumed number of environment sectors (always 1) leads to an
incorrect top_of_range calculation in fw.env.c when a flash device has
an erase block size smaller than the environment data size (number of
environment sectors > 1).
This change updates the default number of environment sectors to at
least cover the size of the environment.
Also corrected a false statement about the number of sectors column in
fw_env.config.
Signed-off-by: Dustin Byford <dustin@cumulusnetworks.com>