Fiddling with Nexus 4 boot image

TLDR; How to modify any system to set ro.debuggable=1 without rebuilding it from source. This setting will make any apk debuggable on the device.

Get the existing boot image off the phone

dd if=/dev/block/mmcblk0p6 of=/mnt/sdcard/boot.img # on the phone
adb pull /mnt/sdcard/boot.img # on your computer

/dev/block/mmcblk0p6 is Nexus 4's boot partition.

Install abootimg from https://github.com/coruus/abootimg. The rest of the process below is stolen from this page.

Extract and unpack initrd

mkdir boot 
cd boot
abootimg -x /tmp/boot.img

mkdir initrd
cd initrd
cat ../initrd.img | gunzip | cpio -vid

Edit default.prop, setting anything you want, including ro.debuggable=1.

Repack initrd and boot image

cd initrd
find . | cpio --create --format='newc' | gzip > ../myinitrd.img
 
cd ..
abootimg --create myboot.img -f bootimg.cfg -k zImage -r myinitrd.img

Flash to phone

adb reboot-bootloader
fastboot flash boot myboot.img