package/uboot-tools: tools/env/fw_env.h: remove env.h

As reported by Nicolas Carrier on the Buildroot mailing list [1],
there is a new build issue while building a program which interacts with
the u-boot environment. This program uses the headers of the ubootenv
library provided by uboot-tools.

This is an upstream change from uboot [2] adding "#include <env.h>" to
fw_env.h. Adding env.h require a board configuration to build.

But only fw_env.h header is installed in the staging directory by
uboot-tools package, but since it now include env.h the build is broken
because env.h is missing from the staging directory.

It's seems an upstream bug since env_set() is not used in fw_env tool.
Nicolas removed env.h from fw_env tool and fixed it's build issue.

This problem is present since uboot v2019.10, so the uboot version
present in Buildroot 2020.02 is affected.

It's probably not a problem for upstream uboot but it's a problem
for uboot-tools package that build uboot tools without a board
configuration for the target.

[1] http://lists.busybox.net/pipermail/buildroot/2020-April/280307.html
[2] https://gitlab.denx.de/u-boot/u-boot/-/commit/9fb625ce05539fe6876a59ce1dcadb76b33c6f6e



Reported-by: Nicolas Carrier <nicolas.carrier@orolia.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[yann.morin.1998@free.fr: add URL to upstream commit]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>