Macbook Pro stuck booting after Sierra 10.12.1 update
my MBP is a Mid-2012 Retina, and I have had this issue with both the HP Printer Update 5.0 and 5.1. I successfully ignored the update for a while, but tried today and ended up with the same problem. Here's a step-by-step fix that helped me get back on the road:

The Fix (if this sounds scary, don't do it - find someone who is comfortable with  this process to help you):


Hold the power button until your machine shuts off.
Hold the 'command' and 'R' keys simultaneously, then start your machine. Wait
for the chimes before you let go. 
You will boot into the recovery partition. 
Open the 'terminal' application (hint: it's in a menu)
cd /Volumes/'your volume name here'/Library/Extensions (to figure out your volume name, type 'cd /Volumes', then type 'ls' to see the list of available volumes. The one you want to use is the one you normally boot into. 
type 'ls - l' when you are in the Extensions directory of your boot Volume, and confirm that you see the directory 'hp_io_enabler.kext' 
type 'rm -rf hp_io_enabler.kext' this will recursively delete the directory and all contents. the f means 'force'. 
if you like, type 'ls -l' to confirm that the 'lil guy is gone. 
quit the Terminal and reboot your machine to the normal Startup.
get on with your life!


To hide specific updates in Sierra, right-click (Command click) on the specific update, and select the convenient 'Hide Update' that appears. How easy is that?