Fix fastbootd denials when using /proc/bootconfig.

Bug: 189493387
Test: fastboot flashall on device using bootconfig
Change-Id: Ibfb7c8a2861f61803a449a4b0ec9ed92ded5c4de
diff --git a/private/fastbootd.te b/private/fastbootd.te
index 0174faa..40b3945 100644
--- a/private/fastbootd.te
+++ b/private/fastbootd.te
@@ -41,4 +41,7 @@
 
   # Mount /metadata to interact with Virtual A/B snapshots.
   allow fastbootd labeledfs:filesystem { mount unmount };
+
+  # Needed for reading boot properties.
+  allow fastbootd proc_bootconfig:file r_file_perms;
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