Add a "single" source/header file generation and build mode (#2790)

Introduce a new way of building: before any file is compiled
all source files are combined into a single C file and all
header files into a single H file (per subdir).

This new approach makes it possible to quickly integrate JerryScript
into other projects:

```
$ gcc -o demo demo.c jerryscript.c jerryscript-port-default.c -lm
```

To use the source generator run:
```
$ cmake -Bbuild_dir -H. -DENABLE_ALL_IN_ONE_SOURCE=ON
$ make -C build_dir generate-single-source
```

This will create the following files in the `build_dir`:
* jerryscript.c
* jerryscript.h
* jerryscript-config.h
* jerryscript-port-default.c
* jerryscript-port-default.h

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.u-szeged@partner.samsung.com

tools/run-tests.py

@@ -160,6 +160,8 @@ JERRY_BUILDOPTIONS = [
             ['--regexp-recursion-limit=1000']),
     Options('buildoption_test-vm_recursion_limit',
             OPTIONS_VM_RECURSION_LIMIT),
+    Options('buildoption_test-single-source',
+            ['--cmake-param=-DENABLE_ALL_IN_ONE_SOURCE=ON']),
 ]
 
 def get_arguments():