Add more detailed information about OpenWRT to debugging section

[Frijol]

• How do you use DTERM with OpenWRT?
• How do you set up wifi using the config files?
• How do you use the sysupgrade command with OpenWRT?
• What does bootloader mode actually look like?
• Where do SSH keys go if you place them manually?

Migrated from @johnnyman727's issue tessel/onboarding#2