Load boot image props.

This change does the following:

- Create /second_stage_resources empty dir at root.
- At runtime:
  - At first stage init:
    - mount tmpfs to /second_stage_resources.
    - Copy /system/etc/ramdisk/build.prop to
      /second_stage_resources/system/etc/ramdisk/build.prop
  - At second stage init:
    - Load prop from the above path
    - umount /second_stage_resources

Test: getprop -Z
Test: getprop
Bug: 169169031

Change-Id: I18b16aa5fd42fa44686c858982a17791b2d43489