Move to qemu sifive unleashed

[ethandmd]

qemu sifive_u board spec (updated on branch in src/hw/param.rs

Migrate:

• [ ] src/hw/param.rs -- @ethandmd
• [ ] Makefile -- @ethandmd
• [ ] CLINT -- @TCCQ
• [ ] PILC - @TCCQ
• [ ] UART -- @ethandmd
• [ ] LINKER (<-- note that DRAM starts at same address?) -- @TCCQ

[TCCQ]

Resource: qemu sifive

[TCCQ]

This should be done with respect to the #57 HAL system, if it is done at all.