elf2image: Add option to specify output filename prefix

This PR adds an option to specify the output filename prefix of the command `elf2image`. Backward compatiblity is ensured as the default naming scheme does not change.

Examples:
* `./esptool.py elf2image -o "build/app-" myfirmware.out`: Creates two files `app-0x00000.bin` and `app-0x40000.bin` in the directory `build` (directories are not created).
* `./esptool.py elf2image -o "" myfirmware.out`: Creates two files `0x00000.bin` and `0x40000.bin` in the current directory.