Partial backport of upstream patch.``

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Maintainer: Jakub Tymejczyk <jakub@tymejczyk.pl>
Compile tested: ramips, Xiaomi Router 3G, fc54256
Run tested: ramips, Xiaomi Router 3G, 0f54d96

Description:
Mosh is "Remote terminal application that allows roaming, supports
intermittent connectivity, and provides intelligent local echo and line
editing of user keystrokes".

Project's site: https://mosh.org

Makefile and patch taken from: https://github.com/mchwalisz/mosh-openwrt
updated by me

Signed-off-by: Jakub Tymejczyk <jakub@tymejczyk.pl>
(Makefile cleanup and size optimizations)
Signed-off-by: Rosen Penev <rosenp@gmail.com>