ci: add multi-arch-build.yml;

[a-gave]

Add a new packages repository at:

• http://github.com/libremesh/lime-feed/tree/gh-pages/master/packages/$ARCH_PACKAGES
• http://feed.libremesh.org/master/packages/$ARCH_PACKAGES

That includes packages precompiled by openwrt-sdk for each specific architecture

This is currently required only for the package shared-state-async
After this merge #1067, imagebuilder builds fail, using the default set of packages, unless you are building for x86_64

update the imagebuilder build guide on README.md
update lime-system/files/etc/uci-defaults/92_add-lime-repos to use also the new repo

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.56%. Comparing base (fe00acb) to head (81ea18b).
Report is 2 commits behind head on master.

❗ Current head 81ea18b differs from pull request most recent head 3e1d0ab. Consider uploading reports for the commit 3e1d0ab to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1091      +/-   ##
==========================================
+ Coverage   79.54%   79.56%   +0.02%     
==========================================
  Files          53       53              
  Lines        4566     4566              
==========================================
+ Hits         3632     3633       +1     
+ Misses        934      933       -1     

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[ilario]

Did not test yet, but we need to have the CI to run this in order to test this. So let's merge and let's see if it works as expected. Thanks @a-gave !!!!!!!!!