well, here's a slight workaround for me...
see, i always keep my phone on vibrate unless it's absolutely necessary for me to have my ringtone on (i know i'm leaving my phone in another room for a few minutes, i'm out in an extremely loud area, etc.)
with that being the case, every time i restart my phone, it doesn't make a boot-up sound because my phone was silent already
now if you feel like you absolutely need your ringtone on, i don't know what to say
but if you're keen enough, just remember to take your phone down to silent/vibrate before you reboot it, then just turn the volume back up after