Previously the Makefile.external file would error out if DMAI_INSTALL_DIR
wasn't set explicitly -- even if it was defined by the DVSDK's Rules.make.
There was a comment indicating that if your Rules.make set DMAI_INSTALL_DIR
disable setting its default value of "undefined", which
the error check.
This commit moves the DMAI_INSTALL_DIR error check until after Rules.make
is included. This way, the error check comprehends if it was set by
and no modification to the Makefile is needed in this case.