Refactor libremesh.mk and makefiles

[spiccinini]

The idea is to be able to use libremesh.mk like a module so that it can be more easily customized.

I still have to test a full build with this, but want to share the idea to get feedback.

Fixes #825

[codecov-io]

Codecov Report

Merging #829 (f262998) into master (19cc376) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #829   +/-   ##
=======================================
  Coverage   72.27%   72.27%           
=======================================
  Files          37       37           
  Lines        3293     3293           
=======================================
  Hits         2380     2380           
  Misses        913      913           

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[G10h4ck]

This looks much better to me!

[spiccinini]

Ok so I've built an image with this changes, so far so good.

[spiccinini]

Ping! @ilario what do you think?

[ilario]

Did not test, looks working to me