So I screwed up the bootloader on my PC. Again.
I was able to fix it but it's always a pain to borrow another computer to download an ISO of my favorite distro and prepare a USB stick to repair that every time.
There's a neat application called DriveDroid that lets you use your phone as a USB drive for this exact purpose, but on my phone it was setting the gadget driver to the wrong mode, causing it to emulate a CDROM drive instead of a more conventional thumb drive (which Arch doesn't like), plus it's not open source, so I made my own application which is much faster and does things properly.
Source code and downloads are on GitHub
(APKs in the releases tab, root is mandatory).
I tested it on my (2012 model) Nexus 7 and my OnePlus 2, not with my Nexus 4 though since it's on the unrooted stock ROM and I need to charge it as well. I've heard of a Nexus 5 and a OnePlus X working fine with it, so I presume compatibility should be fairly good.
Have fun with it.