Allow changing wifi password

[germanferrero]

This PR is rebased with the branch of this PR: #896
Please review that one first.

It adds ubus endpoints for enabling/disabling the ap mode in 2ghz radios.
And for setting up a wifi password for the apname mode in 2ghz radios.

This serves as a backend for a UI in the LimeApp to do the same.

[spiccinini]

Good changes!

[codecov-commenter]

Codecov Report

Merging #901 (1fc11a0) into master (de7f38f) will increase coverage by 0.14%.
The diff coverage is 96.19%.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
+ Coverage   74.71%   74.86%   +0.14%     
==========================================
  Files          43       44       +1     
  Lines        3655     3788     +133     
==========================================
+ Hits         2731     2836     +105     
- Misses        924      952      +28     

Impacted Files	Coverage Δ
...-utils/files/usr/lib/lua/lime/wireless_service.lua	58.25% <95.91%> (ø)
...es/lime-system/files/usr/lib/lua/lime/wireless.lua	97.02% <96.42%> (-0.37%)	⬇️
...ubus-lime-utils/files/usr/lib/lua/lime/upgrade.lua	72.94% <0.00%> (+12.27%)	⬆️

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