Add /second_stage_resources tmpfs.

At build time, the directory is created as an empty directory. At
runtime, init mounts tmpfs at this path to preserve files from first
stage init to second stage init.

Right now, first stage init copies the following file to this tmpfs
before switching root:
- /boot/etc/build.prop -> /second_stage_resources/boot/etc/build.prop

After init property service finishes loading all properties, this tmpfs
is umounted, and this directory is left empty.

Bug: 169169031
Test: run and init loads props properly.
Change-Id: Ic6e62b10d8aec446b51c6bc67fdc2dbc943096ba

private/file_contexts

@@ -30,6 +30,7 @@
 /postinstall        u:object_r:postinstall_mnt_dir:s0
 /postinstall/apex   u:object_r:postinstall_apex_mnt_dir:s0
 /proc               u:object_r:rootfs:s0
+/second_stage_resources u:object_r:tmpfs:s0
 /sys                u:object_r:sysfs:s0
 /apex               u:object_r:apex_mnt_dir:s0