EDK2 image generate stages

Step 1: Process Meta-Files  start from Module Top-level dsc file. Then .fdf files then list of inf files to generate multiple level of makefile
Step 2: Start Compiling with nmake/gunmake recursively to generate COFF/PE32 files then execute GenFW to convert to EFI image files
Step 3. If top-level .dsc file contain define of
            FLASH_DEFINITION = XXXX.fdf
            then will invoke gendfs to generate final image file. If no such definition, step 3 will not go. (For those only hope to generate EFI application or driver, can use -m parameter to specify a XXX.inf which should already be listed in top-level dsc file to only compile that specific module)