The DTB argument to boot_other_hart shouldn't be used in BBL

I screwed up refactoring bbl.

Add the '--enable-print-device-tree' argument

I'm trying to debug some device tree problems while booting Linux and
figured it would be really nice to have access to the device tree while
trying to debug these problems.  I think this might be useful for lots
of people, so I went ahead and cleaned up the code enough that it should
actaully work in most cases.

Add a platform interface

Since we don't have a coherent platform specification right now, we've
ended up with slightly different platforms between spike and our FPGA
dev kit.  For now I've added an autoconf option that allows users to
select the relevant platform.

Some platforms can't boot Linux on all the harts.  This commit allows
platforms to define the set of harts that should be prevented from
booting past BBL.  This is essentially just a new mechanism for defining
the DISABLED_HART_MASK.

SiFive has a different logo, so let platforms override said logo.

SiFive's pk fork is the second one I've had to maintain, and it's a huge
pain because people keep just leaving changes all over the tree.  I want
to introduce an interface that the platform-specific details can live
behind so I don't have to keep doing these painful merges.

We were accidentally advancing the PC by 4, not 2.

h/t Karsten Merker

This fixes RV32 pk.

This matches the behavior of mbabaddr/mtval.

The clint was wiping out information discovered by the plic.
Initialize hart stacks as they are discovered.
Then fill in clint+plic info
Then wake the harts.