sim:epiphany: Add epiphany-elf-sim: frontend for starting simulations.

epiphany-elf-sim is a command line tool written in Python that makes it easy
to start simulations.

It depends on esim and will not be installed unless --enable-emesh-sim
was passed to configure.

See README.epiphany-elf-sim.md for more info.
sim:epiphany:esim: Update documentation
sim:epiphany: CPU specific parts must be in end of _cpu_struct

Everything guarded by WANT_CPU_EPIPHANYBF must be put in the end of the
_sim_cpu struct. Otherwise files that don't set this flag can end up
reading incorrect data, or worse, corrupt the data in the struct by writing to
the wrong offset.