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/library/CMakeLists.txt b/library/CMakeLists.txt
index f3b93fb..18aff5a 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -109,7 +109,7 @@
     ssl_tls13_generic.c
 )
 
-if(DEV_MODE)
+if(GEN_FILES)
     find_package(Perl REQUIRED)
 
     file(GLOB error_headers ${CMAKE_CURRENT_SOURCE_DIR}/../include/mbedtls/*.h)