Replace NULL with nullptr

Change-Id: Iad50be617d318ca98883b843229c960ad5b9afa9
diff --git a/linker/linker_environ.h b/linker/linker_environ.h
index d3f54fd..0f6ac08 100644
--- a/linker/linker_environ.h
+++ b/linker/linker_environ.h
@@ -35,7 +35,7 @@
 extern void linker_env_init(KernelArgumentBlock& args);
 
 // Returns the value of environment variable 'name' if defined and not
-// empty, or NULL otherwise.
+// empty, or null otherwise.
 extern const char* linker_env_get(const char* name);
 
 // Returns the value of this program's AT_SECURE variable.