Add sencha targets - Supports top_sencha as a verilator target. Currently does not support sencha-specific binaries in sw/ - Similar to matcha_hw.mk with the key differences being top_sencha passed as an arg to $TOPS for autogen flow and outputs stored in out/sencha Bypass-Presubmit-Reason: manually tested non-TODO items Change-Id: I4556ef03329130548e2d431fa8e3bbc8d61861bd