we can use uart to debug nuttx,like debugger:
1. read/write memory
2. Use watchpoint,breakpoint,single step.
use up_debugpoint api
3. Ctrl+c to stop, continue, or single step.
hold uart send and receive
4. register a panic event, when crash or assert/panic, we use uart to
debug.
Signed-off-by: anjiahao <anjiahao@xiaomi.com>
