ecli: add btfhub support

[ocfox]

Tested on ubuntu 18.04 kernel: 4.15.0-189-generic x86_64. work fine.

[codecov]

Codecov Report

Merging #159 (2ef4ac7) into master (c47b42b) will decrease coverage by 0.07%.
The diff coverage is 66.66%.

❗ Current head 2ef4ac7 differs from pull request most recent head a2f78bb. Consider uploading reports for the commit a2f78bb to get more accurate results

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   81.35%   81.28%   -0.07%     
==========================================
  Files          25       25              
  Lines        2955     2955              
==========================================
- Hits         2404     2402       -2     
- Misses        551      553       +2     

Impacted Files	Coverage Δ
bpf-loader/src/btf_helpers.c	0.00% <0.00%> (ø)
bpf-loader/src/eunomia_bpf.cpp	65.14% <100.00%> (-0.54%)	⬇️
compiler/cmd/src/config.rs	85.07% <100.00%> (ø)

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[yunwei37]

Please rebase on master branch.

[yunwei37]

Please add a document to describe how to use the btfhub with ecc and ecli.

[ocfox]

I think we should add the changes after migrating the document. I will complete it later. This PR can be merged for now.