Unify htif implementation with firesim

[jerryz123]

The original inheritance structure for our various htif_t implementations was

- htif_t (upstream riscv-tools)
  - firesim_fesvr_t
  - tsi_t (forked with custom mods in esp-isa-sim)

The new inheritance structure is

- htif_t (upstream riscv-tools)
  - tsi_t (upstream riscv-tools)
    - testchip_fesvr_t (adds loadmem support)
      - firesim_fesvr_t

Users no longer have to install esp-tools to get loadmem support in CY SW sims.
Additionally lots of duplicated code between upstream tsi_t and firesim_fesvr_t is removed.

Future new features that are too controversial for upstream tsi_t can be added to testchip_fesvr_t.

Type of change: other enhancement

Impact: software change

Release Notes

[colinschmidt]

LGTM. I assume you tested:

• Verilator with and without loadmem
• VCS with and without loadmem
• Firesim with and without loadmem

[jerryz123]

Yep those all work. For firesim I tested FireSim target SW sim, which should test the htif functionality sufficiently.