kernel: allow rebooting, and writing to /dev/__kmsg__

Addresses the following denials:

  avc:  denied  { write } for  pid=1 comm="init" path=2F6465762F5F5F6B6D73675F5F202864656C6574656429 dev="tmpfs" ino=7214 scontext=u:r:kernel:s0 tcontext=u:object_r:tmpfs:s0 tclass=chr_file permissive=0
  avc:  denied  { write } for  pid=1 comm="init" name="sysrq-trigger" dev="proc" ino=4026534153 scontext=u:r:kernel:s0 tcontext=u:object_r:proc_sysrq:s0 tclass=file permissive=0
  avc:  denied  { sys_boot } for  pid=1 comm="init" capability=22  scontext=u:r:kernel:s0 tcontext=u:r:kernel:s0 tclass=capability permissive=0

(cherrypicked from commit e550e79c)

Change-Id: I46be370d520c4492d97c6ed7ccdc55cc20b22c49