U-d-f uses hardcoded names for kernel and initrd instead of the ones defined in hardware.yaml

[Simon Eisenmann]

i am using the following hardware.yaml:

```
kernel: assets/uImage
dtbs: assets/dtbs
initrd: assets/uInitrd
partition-layout: system-AB
bootloader: u-boot
```

When a device tarball with this yaml is created with u-d-f, the names of the files get changed to vmlinuz and initrd. This behavior is hardcoded in http://bazaar.launchpad.net/~phablet-team/goget-ubuntu-touch/trunk/view/head:/diskimage/common.go#L477

For me this is a regression, as i just have updated to latest u-d-f (0.31-0ubuntu1). In the version 0.20 i used previously the files got created in the target image with the names i have specified in hardware.yaml. The behavior changed in http://bazaar.launchpad.net/~phablet-team/goget-ubuntu-touch/trunk/revision/195 - The previous functionality should be brought back to make sure snappy can properly boot Kernels which need a uboot converted initrd and kernel file.

[Simon Eisenmann (longsleep)]

For reference, https://github.com/longsleep/snappy-odroidc/commit/c9aafdbc13978fffa3e5db0da565eb9d4ae2d7cc is the workaround i had to use to make my images boot with u-d-f after the filenames got hardcoded.

[Michael Vogt (mvo)]

u-d-f got replaced with ubuntu-image so this is fixed now in 16.