Allow user overrides of LINKER_SCRIPT Make variable (#3594)

Makefile: Allow user overrides of LINKER_SCRIPT Make variable

Author: nuket

tools/export/makefile/Makefile.tmpl

@@ -55,7 +55,7 @@ PROJECT := {{name}}
 {% endfor %}
 LIBRARY_PATHS :={% for p in library_paths %} {{user_library_flag}}{{p}} {% endfor %}
 LIBRARIES :={% for lib in libraries %} {{lib}} {% endfor %}
-LINKER_SCRIPT := {{linker_script}}
+LINKER_SCRIPT ?= {{linker_script}}
 {%- block additional_variables -%}{% endblock %}
 
 # Objects and Paths