Allow update_engine to write BCB.

update_engine can trigger a factory-reset when the update to an older
version or an incompatible version requires it.

Bug: 28700985
TEST=Updated a device with a factory-reset required and the BCB was

Change-Id: Ief3dd386a14b669141d75b561122a3095efc0a6f
diff --git a/update_engine.te b/update_engine.te
index 33e8134..3c5bfa1 100644
--- a/update_engine.te
+++ b/update_engine.te
@@ -28,6 +28,11 @@
 allow update_engine boot_block_device:blk_file rw_file_perms;
 allow update_engine system_block_device:blk_file rw_file_perms;
+# Allow to set recovery options in the BCB. Used to trigger factory reset when
+# the update to an older version (channel change) or incompatible version
+# requires it.
+allow update_engine misc_block_device:blk_file rw_file_perms;
 # Don't allow kernel module loading, just silence the logs.
 dontaudit update_engine kernel:system module_request;