rootdir: Allow board specific folders and symlinks in root directory

Building without ramdisk requires a way to specify board specific
directoryies and symlinks in the root directory at build time.

Change-Id: I11301e98228bc4761f3aee177a546146651b9f25
(cherry picked from commit d7549c9a65cad886f672af41f5fca6f0bd0c12fa)