Allow specifying project relative or full paths for elements on the command line

This is a question of being more user friendly to the unsuspecting user, and allowing element targets to be expressed as project relative, or simply as full paths on the command line.

Original report follows:

argument path resolution is illogical

in the following case, /elements/ should not appear twice, surely?

/src> ls
README.md  convert  elements  files  keys  project.conf  scripts
/src> bst build elements/systems/minimal-system.bst
[--:--:--][][] START   Loading pipeline
[00:00:00][][] FAILURE Loading pipeline
Error loading pipeline: Could not find file at /src/./elements/elements/systems/minimal-system.bst