build: Fix extra userdata generation

Image generation has been combined into the target file script and
requires the prefix parameter to work properly. This also needs to be
called properly when creating fastboot packages.

Change-Id: I1f716cc375ffe401d5f18487330fc9bb809ff3b4
Ticket: OPO-191

build: Fix extra userdata generation some more

Use the prefix variable when placing the file in the target files
package.

Change-Id: Ia4b795416b58c493c3ea91b964e3b6f4cdf0fef1
Ticket: OPO-191