Allow editing Pollen policy (for real)

[liujustin604]

Is your feature request related to a problem? Please describe.
Currently the editing pollen feature doesn't actually work, because the rootfs is mounted read only so saving the file always fails.

Describe the solution you'd like
the policy file would be stored in stateful somewhere (perhaps /mnt/stateful_partition/murkmod/pollen/policy.json) instead of being builtin to the image by image_patcher.sh

At boot time, chromeos_startup.sh creates a bind mount over /etc/opt to /tmp/overlay/etc/opt
(the same way that the original pollen works without rootfs verification removed)

then copies the pollen file over to /tmp/overlay/etc/opt/chrome/policies/managed/policy.json

the mush shell's 13) Edit Pollen feature should be updated to write to the stateful, and then copy the file over to the tmp directory

However

if the rootfs is not mounted as RO while chromeos_startup runs, we can instead just copy the file from stateful to the rootfs directly (I say this because chromeos_startup is able to create the file "rootkey" in the rootfs)

Would this work better as a plugin or a built-in feature of murkmod?
builtin feature

[rainestorme]

Should be easy enough to implement. One sec...

[rainestorme]

Should be fixed in 39854fd. Give it a try and let me know if it works.