boot.d: "userinit" venirified

adds both ROM utility and user customizability
1. instead of polling in a backgrounded shell, post-boot init.d scripts can now be stuck in /system/etc/boot.d
2. after the boot.d scripts run, any crap a user found on XDA and ADB pushed to /data/boot.d will run (and probably brick their phone. TROLOLOL newbs)

Change-Id: I3d31d028777f2dfc42c53950a61d69fa5fc75de4