cmd/devp2p: use bootnodes as crawl input

[ucwong]

This PR makes the tool use the --bootnodes list as the input to devp2p crawl. The flag will take effect if the input/output.json file is missing or empty.

[codecov]

Codecov Report

Merging #1732 (6b0a37a) into master (425d290) will decrease coverage by 0.02%.
Report is 2 commits behind head on master.
The diff coverage is 0.00%.

❗ Current head 6b0a37a differs from pull request most recent head 9faa058. Consider uploading reports for the commit 9faa058 to get more accurate results

@@            Coverage Diff             @@
##           master    #1732      +/-   ##
==========================================
- Coverage   52.75%   52.74%   -0.02%     
==========================================
  Files         485      485              
  Lines       60417    60427      +10     
==========================================
- Hits        31876    31874       -2     
- Misses      26093    26114      +21     
+ Partials     2448     2439       -9     

Files Changed	Coverage Δ
cmd/devp2p/crawl.go	0.00% <0.00%> (ø)
cmd/devp2p/discv4cmd.go	0.00% <0.00%> (ø)
cmd/devp2p/discv5cmd.go	0.00% <0.00%> (ø)

... and 15 files with indirect coverage changes