- remove Spike/BBL support in favour of Qemu (>=4.2.1)
- add 'riscv_qemu' board, remove 'spike' board'
- update to privileged ISA v1.10 (from v1.9.1)
- use direct system calls for privileged core threads (they call into
the kernel and don't use mode changing system calls, i.e. 'ecall',
semantics)
- use 'OpenSBI' semtantics for SBI calls (to machine mode) instead of
BBL
issue #4012