scons: Add sim_object and enums arguments to SimObject().

This will explicitly declare what SimObject and Enum types need to be set
up in C++, which will make importing all the SimObject modules during
the setup phase of SCons uneccessary.

Change-Id: Id2d7603daf33b236ceaa0789e2f089f589d34e62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49406
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/net/SConscript

@@ -40,7 +40,11 @@
 
 Import('*')
 
-SimObject('Ethernet.py')
+SimObject('Ethernet.py', sim_objects=[
+    'EtherLink', 'DistEtherLink', 'EtherBus', 'EtherSwitch', 'EtherTapBase',
+    'EtherTapStub', 'EtherDump', 'EtherDevice', 'IGbE', 'EtherDevBase',
+    'NSGigE', 'Sinic'] +
+    (['EtherTap'] if env['HAVE_TUNTAP'] else []))
 
 # Basic Ethernet infrastructure
 Source('etherbus.cc')