u-d-f failed when device booted

[Alex Tu]

What you expected to happen:
u-d-f can work when device booted.

What happened instead:
u-d-f only work in fastboot or recovery.
If execute u-d-f when device booted to ubuntu image, it always failed with these message:

2015/03/30 15:32:55 Start pushing /home/alextu/.cache/ubuntuimages/gpg/image-signing.tar.xz to device
2015/03/30 15:41:17 Cannot push /home/alextu/.cache/ubuntuimages/pool/custom.tar.xz.asc to device: free space on /cache/recovery is 664M

Tested devices: Krillin, Arale (But it works on Nexus 4)
Channel: ubuntu-touch/vivid-proposed
Package ver: ubuntu-device-flash 0.16-0ubuntu1
Host: Trusty