fix cmake bug (apache#21)

Author: tmoreau89

cmake/modules/VTA.cmake

@@ -39,8 +39,10 @@ elseif(PYTHON)
   endif(APPLE)
 
   # PYNQ rules for pynq v2.3
-  find_library(__cma_lib NAMES cma PATH /usr/lib)
-  target_link_libraries(vta ${__cma_lib})
+  if (${VTA_TARGET} STREQUAL "pynq" OR ${VTA_TARGET} STREQUAL "ultra96")
+    find_library(__cma_lib NAMES cma PATH /usr/lib)
+    target_link_libraries(vta ${__cma_lib})
+  endif()
 else()
   message(STATUS "Cannot found python in env, VTA build is skipped..")
 endif()