Add Target to Generate Top-of-project-relative Manifest File

[gerickson]

This proposes to add support for a target manifest that creates a project manifest file, default name MANIFEST, containing the paths of all project DISTFILES (as would be contained by executing make dist) relative to the top of the project tree.

The default manifest name could be overridden with make MANIFESTFILE=<name> manifest.

[gerickson]

/cc/ @rwalker-com

[rwalker-apple]

so sweet.

[gerickson]

Work in progress at https://github.com/nestlabs/nlbuild-autotools/tree/user/gerickson/make-manifest