fw_env: calculate default number of env sectors

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>