Bug#986361: libubootenv-tool: Add missing manpages

[Bastian Germann]

Package: libubootenv-tool
Severity: normal

Please add manpages for fw_printenv and fw_setenv. You can base them on the former u-boot-tools' 
manpages:

https://manpages.debian.org/buster/u-boot-tools/fw_printenv.8.en.gz
https://manpages.debian.org/buster/u-boot-tools/fw_setenv.8.en.gz