sh: Rip out special unknown machvec.

This kills off the BareCPU board as a "special" machvec, rather,
we leave this as a default for when no other vector is available,
or when we want to use it in combination with other vectors for
testing with generic ops. As sh_mv is copied out anyways (or
overloaded when an alternate vector is explicitly selected), this
doesn't consume any additional memory.

The generic machvec can be forcibly selected with sh_mv=generic,
or by not having any other boards enabled.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>