Add support for copy_files as defined in HardwarePack v3

[pfalcon 2012-08-27] Would be nice to clarify where files for copy_files are taken from in https://wiki.linaro.org/HardwarePacksV3. Based on the context, they come from package defined by "package" field, but making it explicit is good.
[pfalcon 2012-08-28] Checking with Ryan/Ricardo, we need to support taking files from arbitrary packages, not just main bootloader package (usacase: suppose there's single generic bootloader package, and packages with per-board configs). Also, need to put files not just flat at /boot, but support directory structure (usecase: Grub expects stuff in /boot/grub).
[pfalcon 2012-08-28] Spotted issues with supporting multiple bootloaders in single hwpack (on linaro-hwpack-create level, l-m-c may as well support them).
[pfalcon 2012-08-28] WIP branch: https://code.launchpad.net/~pfalcon/linaro-image-tools/copy_files-lhc . Implements copy_files, as well as proper separation of multiple global and board-specific bootloaders. Need more polishing and tests.
[pfalcon 2012-09-04] There was no copy_files in v2, so no need to convert.

Meta:
Headline: The HardwarePacks v3 format now implements the "copy_files" directive, a list of files to copy from an optionally named packages to an optionally named destination.
Acceptance: copy_files is implemented in the manner enough to support UEFI-enabled images.

Work items:
[pfalcon] Contact Ryan Harkin & Ricardo Salveti for requirements and usecases: DONE
Finalize extended copy_files filespec format (allowing to take a file from any package, and put at arbitrary dest path): DONE
[dooferlad] Refactor tests: DONE
[pfalcon] Implement copy_files support in linaro-hwpack-create: DONE
[pfalcon] Implement copy_files support in linaro-media-create: DONE
[dooferlad] Implement extended copy_files filespec format: DONE
[dooferlad] Revamp multiple bootloaders support: DONE
[pfalcon] Check the need for v2 conversion for copy_files: DONE
[dooferlad] Update Wiki to show new spec: DONE