Makefile: use $LEGATO instead of $NO_BUILD_LEGATO
Makefile: use $LEGATO instead of $NO_BUILD_LEGATO

The mangOH makefile now reads a $LEGATO variable to decide if the Legato
framework at $LEGATO_ROOT should be built.  Previously, but variable
$NO_BUILD_LEGATO was used, but this was a bit verbose.

mangOH/mangOH,mangOH/mangOH,mangOH/mangOH,mangOH/mangOH