Don't use a separate tmpfs mount for /tmp and /run

We don't need a separate tmpfs on /tmp and /run, as we're already
providing a completely in-ram filesystem.

This requires us to override fstab, and device table - these are just
the buildroot standard ones, but with /tmp and /run removed.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/openpower/overlay/etc/fstab b/openpower/overlay/etc/fstab
new file mode 100644
index 0000000..d373dc6
--- /dev/null
+++ b/openpower/overlay/etc/fstab
@@ -0,0 +1,6 @@
+# <file system>	<mount pt>	<type>	<options>	<dump>	<pass>
+/dev/root	/		ext2	rw,noauto	0	1
+proc		/proc		proc	defaults	0	0
+devpts		/dev/pts	devpts	defaults,gid=5,mode=620	0	0
+tmpfs		/dev/shm	tmpfs	mode=0777	0	0
+sysfs		/sys		sysfs	defaults	0	0