Create boot_block_device and allow install_recovery read access

The install_recovery script creates a new recovery image based
off of the boot image plus a patch on /system. We need to allow
read access to the boot image to allow the patching to succeed,
otherwise OTAs are broken.

Addresses the following denial:

  type=1400 audit(9109404.519:6): avc: denied { read } for pid=341 comm="applypatch" name="mmcblk0p37" dev="tmpfs" ino=9186 scontext=u:r:install_recovery:s0 tcontext=u:object_r:block_device:s0 tclass=blk_file permissive=0

TODO: Add device specific labels for the boot image.

Bug: 19534538
Change-Id: Ic811ec03e235df3b1bfca9b0a65e23307cd968aa