Partial verilator support.

verilator verif/verilator_toplevel.v --cc -DUSE_DEBUG_UART=1
../BUILD/verif/keynsham_defines.v ../BUILD/verif/oldland_defines.v
-Irtl/keynsham -Irtl/oldland -Irtl/common -Irtl/spimaster
-Irtl/sdram/rtl -Iverif
jtag: optimize request getting.

Use the more_data flag to determine if we need to get a request rather
than a syscall.
devicemodels: make into a library.

This will be used for linking the verilator model.
verif: split the icarus and verilator simulations.
verilator: integrate into build system.
verif: add a wrapper script for the verilator simulation.
oldland-test: add the verilator model.

Also adds --quick to disable the Icarus simulation which takes ~20
seconds, compared to the sub-1 second times of the other simulations.
verilator: factor out the debug interface.
verilator: add full simuart support.
verilator: SPI SD card support.
verif: don't dereference NULL SD card.
docs: document the new simulator.