nestedvm.cmake: fix accidental use of dynamic scope.

In this platform's set_platform_puzzle_target_properties, I referred
to ${EXENAME} twice, which is not one of the function parameters. It
worked anyway, because CMake has dynamic scope, and that variable was
defined - to the right value - within the local-variable scope of the
calling function. But that wasn't at all what I meant to do!

Renamed it to ${TARGET}, which is the actual parameter name we get
passed.

cmake/platforms/nestedvm.cmake

@@ -39,10 +39,10 @@ function(set_platform_puzzle_target_properties NAME TARGET)
     COMMAND ${Java_JAVA_EXECUTABLE}
       -cp ${NESTEDVM}/build:${NESTEDVM}/upstream/build/classgen/build
       org.ibex.nestedvm.Compiler -outformat class -d .
-      PuzzleEngine ${CMAKE_CURRENT_BINARY_DIR}/${EXENAME}
+      PuzzleEngine ${CMAKE_CURRENT_BINARY_DIR}/${TARGET}
     DEPENDS
       ${build_subdir}
-      ${CMAKE_CURRENT_BINARY_DIR}/${EXENAME}
+      ${CMAKE_CURRENT_BINARY_DIR}/${TARGET}
     WORKING_DIRECTORY ${build_subdir})
 
   add_custom_target(${TARGET}-jar ALL