zero-extend RV32 physical addresses for HTIF

Refactor pk, bbl, machine into separate libraries

Yuck.

Disentangle PK and BBL a bit

Remove SBI HTIF support; add console_getchar

Fix struct stat size mismatch for RV32

Split pk functionality into pk and bbl

pk is now an AEE only (i.e. it can only execute user programs).
bbl is now an SEE only (i.e. it can only host kernels).

Update to new privileged spec

Support linkat, unlinkat, mkdirat syscalls

Also, old-style syscalls e.g. open are now implemented with e.g. openat.

Add several syscalls, and move syscall header file from newlib
syscalls added (openat, access, accessat, faccessat, etc.)

Abandon MIPS convention for indicating syscall errors in register a3

add BSD license

temporary undoing of renaming

Renamed packages

[pk,fesvr] improved proxykernel build system

Now uses a modified MCPPBS. Add --host=riscv to configure path.

Front-end server now just searches PATH for riscv-pk, so just install the pk
to somewhere in your path.