A new GAIA_OUTOFTREE_APP_SRCDIRS variable is introduced that contains a list
of app directories outside of the Gaia root that should be included in the
build.  The apps found within GAIA_OUTOFTREE_APP_SRCDIRS are symlinked to an
outoftree_apps/ directory within the Gaia root.

Change-Id: I903a904485003e27ff69779debb67a918dd39b4b