x86, um: get rid of arch/um/os symlink

we can get DEV_NULL defined for arch/um/drivers/null.c in less
convoluted ways, TYVM...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/um/drivers/Makefile b/arch/um/drivers/Makefile
index d283e7b..1d9b6ae 100644
--- a/arch/um/drivers/Makefile
+++ b/arch/um/drivers/Makefile
@@ -62,5 +62,6 @@
 
 # pcap_user.o must be added explicitly.
 USER_OBJS := fd.o null.o pty.o tty.o xterm.o slip_common.o pcap_user.o vde_user.o
+CFLAGS_null.o = -DDEV_NULL=$(DEV_NULL_PATH)
 
 include arch/um/scripts/Makefile.rules