fw_env: calculate default number of env sectors

Patch from U-Boot to update the environment
(commit 23869bf80b68af9ef1802b9e556f4fd0e9e57fe5)

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.

Signed-off-by: Dustin Byford <dustin@cumulusnetworks.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>
Build fw_env together with u-boot handler

Signed-off-by: Stefano Babic <sbabic@denx.de>