kdb: support new lines without carriage returns

kdb expects carriage returns through the serial port to terminate
commands.  Modify it to accept the first seen carriage return or
new line as a terminator, but not treat \r\n as two terminators.

Change-Id: I06166017e7703d24310eefcb71c3a7d427088db7
Signed-off-by: Colin Cross <ccross@android.com>