Rename DEV_MODE to GEN_FILES

GEN_FILES is a bit clearer as it describes what the setting
does more precisely.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/programs/test/CMakeLists.txt b/programs/test/CMakeLists.txt
index 94331b8..142a831 100644
--- a/programs/test/CMakeLists.txt
+++ b/programs/test/CMakeLists.txt
@@ -27,7 +27,7 @@
     target_link_libraries(cpp_dummy_build ${mbedcrypto_target})
 endif()
 
-if(DEV_MODE)
+if(GEN_FILES)
     find_package(Perl REQUIRED)
 
     add_custom_command(