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
written.

Change-Id: Ief3dd386a14b669141d75b561122a3095efc0a6f

update_engine.te

@@ -28,6 +28,11 @@ allow update_engine block_device:dir search;
 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;