debug: tweak debug device configurations

1. remove source when no debug device exists
2. tweak function prototype

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
debug: add function to change debug device

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
debug: add a simple ram debug device implementation

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
Merge pull request #59 from vh21/tweak-dbg

Tweak debug device implementation