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 and feel free to ask anything! Thanks.

(not sure what practices are regarding ccing here, sorry if it's out place. Last few names who pop up for mtd-utils in git log are: @mps @kaniini )