mtd-utils: add mtd-utils-dev subpackage

mtd-utils does not automatically install its headers/lib, but some projects like swupdate need them. Generate a new mtd-utils-dev package with these like debian does

note: I'm not aware of any other package that needs this (checking reverse build dependencies on debian), so I assume other tools like swupdate would just embed the mtd-utils sources... (debian package name: libmtd-dev) It might be better to first suggest to mtd-utils upstream to install these files first?

Well, either way I'd rather have a core package updated than have to maintain a slightly different version for our needs here, so please consider it! Thanks.