init-storage: Support custom writable paths in /etc

Custom builds of Aimee OS can now specify additional paths under `/etc`
that should be writable.  This is accomplished by populating a file
named `/etc/aimee-os/writable-etc` with a list of paths.  Each line must
indicate the type of file (regular file: `f`, directory: `d`) and the
*relative* path under `/etc`.

repos/aimee-os/sec-policy/selinux-aimee-os/files/aimee-os.te

@@ -86,8 +86,10 @@ auth_manage_shadow(aimee_storinit_t)
 auth_relabel_shadow(aimee_storinit_t)
 gen_require(`
 	type shadow_t;
+	attribute configfile;
 ')
 allow aimee_storinit_t shadow_t:file mounton;
+allow aimee_storinit_t configfile:{file dir} mounton;
 
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