console: Add semihosting console

Many chips have only one serial port. When the serial port is occupied
by other devices, there is a lack of a console to output debugging
information. Semihosting can provide a console. The disadvantage of
semihosting is that a debugger must be connected, so it can only be
used for online debugging.

Signed-off-by: ZhongYao Luo <LuoZhongYao@gmail.com>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>