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 https://github.com/libremesh/lime-packages/pull/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 :white_check_mark:

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

:exclamation: 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

```diff @@ 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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