Document and improve the single-source generation (#2986)

Changes done:
* Added usage/configuration info for this mode.
* Created `tools/srcgenerator.py` to allow source/header generation
  without using CMake.
* Adapted CMake to use the `srcgenerator.py` script.
* Added jerry-libm single-source build.
* Improved the `srcmerger.py` script to correctly handle the line numbering.

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

CMakeLists.txt

@@ -231,6 +231,9 @@ if(DEFINED EXTERNAL_LINKER_FLAGS)
   jerry_add_link_flags(${EXTERNAL_LINKER_FLAGS})
 endif()
 
+# Used for placeholder to attach single source build targets
+add_custom_target(generate-single-source)
+
 # Jerry's libm
 if(JERRY_LIBM)
   add_subdirectory(jerry-libm)