lib: divide stdio port from debug port

The choice of CONFIG_STDIO_xxxx is used to select STDIO port.
There are 2 choices now,
 1. CONFIG_STDIO_USE_DBGPPORT: use debug uart as stdio device
 2. CONFIG_STDIO_NODEV: null stdio device.

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
debug: make debug port selectable

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>