Commit trunk@611 added support for the new DSPLink directory structure in
Makefile.common, but didn't add it to Makefile.external. Makefile.external
support is also needed when building the plugin separately without building
all of the other dependent components.
Historically DSPLink has contained a "packages" sub-directory
that is the XDC
root of the component. In the latest versions this "packages"
been removed. The build system now adds a variable LINK_XDC_ROOT to indicate
the XDC root of DSPLink that can be set on a per-target basis, and no longer
assumes it is the same as LINK_INSTALL_DIR.