Absolute paths in manifest file is problematic

[netrounds-fredrik]

In the manifest file, the packages.install and packages.trusted-keys requires an absolute path to files. This is problematic since it makes it impossible to perform the build from any directory without first modifying the manifest file.

Quickly looking at the code, both InstallPackages and InstallTrustedKeys seems like they should be able to handle relative paths, as well as absolute paths.

To be able to distinguish a package name from a relative path in packages.install, I suggest using the same method as apt-get install, where relative paths starts with ./.

[andsens]

Sounds like a great idea. Relative paths have been introduced in other parts of the manifest as well. We should stay with the practice of making them relative to the manifest as done in #341.