dev: Refactor UART->Terminal interface

The UART models currently assume that they are always wired to a
terminal. While true at the moment, this isn't necessarily a valid
assumption. This change introduces the SerialDevice class that defines
the interface for serial devices. Currently, Terminal is the only
class that implements this interface.

Change-Id: I74fefafbbaf5ac1ec0d4ec0b5a0f4b246fdad305
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4289
Reviewed-by: Gabe Black <gabeblack@google.com>

src/dev/SConscript

@@ -44,6 +44,7 @@ if env['TARGET_ISA'] == 'null':
 
 SimObject('BadDevice.py')
 SimObject('Platform.py')
+SimObject('Serial.py')
 SimObject('Terminal.py')
 SimObject('Uart.py')
 
@@ -53,6 +54,7 @@ Source('mc146818.cc')
 Source('pixelpump.cc')
 Source('platform.cc')
 Source('ps2.cc')
+Source('serial.cc')
 Source('terminal.cc')
 Source('uart.cc')
 Source('uart8250.cc')