Split out common pcnet code

The core pcnet emulation code is used by both the PCI "pcnet" device
and the SPARC "lance" device.  Split the common code frm the PCI code so
that that can be configures independantly.

Signed-off-by: Paul Brook <paul@codesourcery.com>
diff --git a/default-configs/sparc-softmmu.mak b/default-configs/sparc-softmmu.mak
index 436d2a6..b0310c5 100644
--- a/default-configs/sparc-softmmu.mak
+++ b/default-configs/sparc-softmmu.mak
@@ -7,3 +7,4 @@
 CONFIG_PTIMER=y
 CONFIG_FDC=y
 CONFIG_EMPTY_SLOT=y
+CONFIG_PCNET_COMMON=y