Change index format to differentiate upload commands by OS

[silvanocerza]

The uploader.command field in the generated module_firmware_index.json has been changed to handle some cases like different executable names for different OSs.

"uploader.command": "\"{uploader}\" -i -d --port={serial.port.file} -U true -i -e -w -v \"{loader.sketch}.bin\" -R"

becomes

"uploader.command": {
    "linux": "\"{tool_dir}/bossac\" -i -d --port={serial.port.file} -U true -i -e -w -v \"{loader.sketch}.bin\" -R",
    "windows": "\"{tool_dir}\\bossac.exe\" -i -d --port={serial.port.file} -U true -i -e -w -v \"{loader.sketch}.bin\" -R"
}

"uploader.command": "\"{uploader}\" \"-C{config.path}\" -v  -patmega4809 -cxplainedmini_updi -Pusb  -b115200 -e -D \"-Uflash:w:{loader.sketch}.hex:i\" \"-Ufuse2:w:0x01:m\" \"-Ufuse5:w:0xC9:m\" \"-Ufuse8:w:0x02:m\" "

becomes

"uploader.command": {
    "linux": "\"{tool_dir}/bin/avrdude\" \"-C{tool_dir}/etc/avrdude.conf\" -v  -patmega4809 -cxplainedmini_updi -Pusb  -b115200 -e -D \"-Uflash:w:{loader.sketch}.hex:i\" \"-Ufuse2:w:0x01:m\" \"-Ufuse5:w:0xC9:m\" \"-Ufuse8:w:0x02:m\" "
}

The internal data structures have been updated accordingly.