Allow building netfront and blkfront as modules

diff -r abbe3df33774 linux-2.6-xen-sparse/arch/xen/Kconfig
@@ -94,7 +94,7 @@
           Say Y if you want this feature.
 
 config XEN_BLKDEV_FRONTEND
-	bool "Block-device frontend driver"
+	tristate "Block-device frontend driver"
 	default y
 	help
 	  The block-device frontend driver allows the kernel to access block

@@ -103,7 +103,7 @@
 	  (domain 0), then you almost certainly want to say Y here.
 
 config XEN_NETDEV_FRONTEND
-	bool "Network-device frontend driver"
+	tristate "Network-device frontend driver"
 	default y
 	help
 	  The network-device frontend driver allows the kernel to access

diff -r abbe3df33774 linux-2.6-xen-sparse/drivers/xen/blkfront/Makefile
@@ -1,3 +1,5 @@
 
-obj-y	:= blkfront.o vbd.o
+obj-$(CONFIG_XEN_BLKDEV_FRONTEND)	:= xenblk.o
 
+xenblk-objs := blkfront.o vbd.o
+

diff -r abbe3df33774 linux-2.6-xen-sparse/drivers/xen/netfront/Makefile
@@ -1,2 +1,4 @@
 
-obj-y	:= netfront.o
+obj-$(CONFIG_XEN_NETDEV_FRONTEND)	:= xennet.o
+
+xennet-objs := netfront.o