Add command to erase all flash

[jnross]

In some of our workflows we're still using esptool.py for its erase_flash command. Sometimes we can avoid this with the --erase-parts option on the flash subcommand, but it's awkward that we need to provide an ELF binary to flash when we're only trying to erase data.

I propose that an erase subcommand be added, with options to erase all flash, or to erase only named partitions.