mkfs is required, and must be a path to the zircon mkfs-msdosfs host tool.
Size and offset are optional. They are most useful when writing into an existing disk image or block device, such as when writing into a GPT.
The -ramdisk and -efi-bootloader are optional.
A -manifest option supports dst=src\n formats and all files will be added. This is most useful for targets that require extra files, such as arm targets with extra firmware.