Fixes for included cores

[KrystalDelusion]

• [x] Include nerv as a subtree for more easily testing changes side by side.
• [x] Updates for checking PicoRV32 CSRs
• [x] Updates for latest SERV, see also #9
• [x] Disable failing serv liveness check
• [x] Fix sorting functionality
• [x] Fix #12
• [x] Fix #13
• [x] Fix Jenkins issues with nerv subtree
• [x] Update csr_spec generation with masks for reserved bits
• [x] Replace non working hpm checking in upcnt with a working cover check