Add "elf2image" command to produce flash segments directly from ELF binary.

This requires binutils for Xtensa architecture (commands xtensa-lx106-elf-nm
& xtensa-lx106-elf-objcopy).
Merge pull request #6 from pfalcon/elf2image

Add "elf2image" command to produce flash segments directly from ELF binary.
Merge branch 'master' of https://github.com/themadinventor/esptool