Refresh-v0.6 release candidate 3

Update Rocket to Mar 2018 vintage, Support bare-metal gdb debugging over USB-JTAG, support access to Debian-rv64GC, update to recent kernel, double speed to 50MHz, add 8-packet Ethernet buffer, support DHCP in boot loader and 10X speedup, widen I/O bus to 64-bits, incorporate UART TX/RX fifos, support colour console with keyboard up/down code PC compatibility.