Bug 1007568 - Follow symlinks when copying apps into STAGE_DIR, r=yurenju

Directories under GAIA_APPDIRS might contain symlinks
(e.g. outoftree_apps). When copying the symlinked apps
into STAGE_DIR without -L option, clear-stage-app
will remove the files in the target directory.
Merge pull request #19384 from lhadji/1007568/copy-symlink

Bug 1007568 - Follow symlinks when copying apps into STAGE_DIR, r=yurenj...