do_mounts: Fix CONFIG_BLK_DEV_MD=m case

We don't really have a nice way to say "compile this when CONFIG_FOO
is y, don't otherwise".

Alternatives are:

obj-$(subst m,,$(CONFIG_FOO)) := foo.o

or

obj-$(CONFIG_FOO)	:= foo.o
obj-m :=

or

obj-y			:= do_foo.o
do_foo-$(CONFIG_FOO)	:= foo.o

I chose the last one, though I'm not particularly happy with either.

init/Makefile

@@ -2,10 +2,11 @@
 # Makefile for the linux kernel.
 #
 
-obj-y			:= main.o version.o do_mounts.o initramfs.o
-obj-$(CONFIG_DEVFS_FS)		+= do_mounts_devfs.o
-obj-$(CONFIG_BLK_DEV_RAM)	+= do_mounts_rd.o
-obj-$(CONFIG_BLK_DEV_MD)	+= do_mounts_md.o
+obj-y				:= main.o version.o mounts.o initramfs.o
+mounts-y			:= do_mounts.o
+mounts-$(CONFIG_DEVFS_FS)	+= do_mounts_devfs.o
+mounts-$(CONFIG_BLK_DEV_RAM)	+= do_mounts_rd.o
+mounts-$(CONFIG_BLK_DEV_MD)	+= do_mounts_md.o
 
 # files to be removed upon make clean
 clean-files := ../include/linux/compile.h