The qemu-rv use a small init code for M mode in kernel build.
It is hard-coding and is difficult to change. Due to the fact,
introduce a already mature SBI implement (e.g OpenSBI) to
replace existing code is a better choice.
This patch introduce some change for qemu-rv:
1. use SSTC to provide time interrupt in kernel build
2. remove uncessary M mode trap.
For simplicity, this patch does not add support for booting
nuttx for any core, but force boot core to start core 0 and
let core 0 do the initialization.
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
