QEMU support

[ivmarkov]

Supporting QEMU (that is, the Espressif fork) really boils down to:

• Ability to convert an ELF to BIN (see the other issue - "elf2image" subcommand in espflash #78 )
• Ability to merge bootloader.bin, partitions.bin and the application BIN into a single BIN, as done here

Given that bootloader.bin and the single-factory-app version of partitions.bin that we need for QEMU are already "embedded" in espflash, that extension might be quite a minor one...