bbl: redirect traps to HS mode

Add ns16750 driver for ariane.

Properly license all nontrivial files

mtrap: add a halt IPI used for poweroff (#86)

Otherwise, linux complains the moment an interrupt arrives and
wakes up one of the not-looping cores.

mtrap: loop forever, really

gcc sometimes takes liberties with optimizing away our important halt function!

Add a 16550 UART driver to back the SBI console

QEMU currently provides the console via HTIF and the SBI. That's a bit
messy because BBL polls for serial input, which means that typing too
quickly loses characters. While QEMU has a standard 16550 device model,
there's no way to have two consoles share the console in QEMU (as they'd
step all over each other) so that means we can't have both the HTIF
console and the 16550 console.

With this patch, QEMU can be changed to use a 16650 instead of the HTIF
for serial output. Linux will use the SBI for early printk support
(which is fine, polling for output is stable) and then swap over as soon
as it detects the UART. When Linux swaps it prints out the whole
history, but there's probably a way to get around that. There's a few
lines that are output to both, but it appears the Linux driver is close
enough to ours that nothing catastrophic happens -- there's not much to
the device, so hopefully that pans out on real hardware too.

Once Linux swaps over to natively using the driver we get reliable
console input. If you don't have the in-kernel driver then Linux never
swaps over and keeps using the SBI console just like before.

Remove the platform interface

We now automatically detect everything that the platform interface used
to be used for, so it's now obsolete!

Detect harts that can't boot Linux instead of hard-coding them

This checks to see if a hart can't boot Linux by looking for a
compatible "mmu-type" field. If the hart can't boot Linux, then bbl
masks it off.

finisher: support terminating sifive devices simulation (#61)

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.

Move DISABLED_HART_MASK to the 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.

Allow the platform to disable HTIF

Always write sbadaddr on trap redirection

Remove num_harts; use hart_mask exclusively

Rename HART_MASK to DISABLED_HART_MASK to clarify polarity

bbl: prevent named cores from booting

uart: add physical device driver

Separate page faults from physical memory access exceptions

Simplify interrupt-stack discipline

https://github.com/riscv/riscv-isa-manual/commit/f2ed45b1791bb602657adc2ea9ab5fc409c62542

WIP on SBI

Handle IPIs and timer interrupts more quickly

WIP towards ECALL interface for SBI

Cleanly separate HTIF code; don't poll keyboard on timer interrupt

Incorporate sptbr/sfence.vma changes

avoid non-standard predefined macros

Acquire lock before attempting tohost/fromhost sequences

add htif section in linker script, with ALIGNs to prevent MMIO and data from being placed on same page (#32)

Improve trap redirection code

Clear IPIs using MMIO, not mip CSR

Poll HTIF console on timer interrupt

Find IPI address in configuration string

Don't use tohost/fromhost registers; communicate with host via memory

Remove mtime/mtimecmp

Remove dead code

Set time comparator correctly on RV32

The old code truncated the upper 32 bits, and even if it got that right,
it would have generated spurious interrupts.

Refactor pk, bbl, machine into separate libraries

Yuck.