Make lguest compile with CONFIG_BLOCK=n and CONFIG_NET=n

Gabriel C reports lguest doesn't compile with CONFIG_BLOCK=n.  Fix this
by introducing a config var for the block device, which depends on
LGUEST && BLOCK.  Do the same for the net driver, rather then depending
gratuitously on CONFIG_NET.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Gabriel C <nix.or.die@googlemail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/block/Makefile b/drivers/block/Makefile
index a7a0990..014e721 100644
--- a/drivers/block/Makefile
+++ b/drivers/block/Makefile
@@ -31,4 +31,4 @@
 obj-$(CONFIG_BLK_DEV_UB)	+= ub.o
 
 obj-$(CONFIG_XEN_BLKDEV_FRONTEND)	+= xen-blkfront.o
-obj-$(CONFIG_LGUEST_GUEST)	+= lguest_blk.o
+obj-$(CONFIG_LGUEST_BLOCK)	+= lguest_blk.o