BOOMv2 (2.2.0)

This marks BOOM version 2.2.0.

Changes:

• All configurations now support RISC-V compressed ("RVC") instructions
• PMP registers are enabled
• The NBDCache has been forked from rocket-chip
• Minor CSR instruction fixes
• ReadTheDocs documentation available at https://docs.boom-core.org/en/latest/
• CircleCI continuous integration
• Switch to Chisel3 syntax
• Updates for September 2018 rocket-chip

This version should be used with the following commit of rocket-chip (from the master branch):
chipsalliance/rocket-chip@50bb13d

This version should be used with the following commit of boom-template (from the master branch):
esperantotech/boom-template@1a0f955