SKR 1.3 support

{
    "$schema": "../board-definition.schema.json",
    "name": "SKR v1.3",
    "manufacturer": "BIGTREETECH",
    "serialPath": "/dev/btt-skr-13",
    "firmwareBinaryName": "firmware-btt-skr-13.bin",
    "compileScript": "compile.sh",
    "flashScript": "flash.sh",
    "flashInstructions": "Flashing only works if there's a properly formatted (FAT16 or FAT32 with a small blocksize) micro SD card in the SKR Pro's SD Card slot. The smaller the SD card the better, a cheap 128mb TF-card is perfect.",
}

{
    "$schema": "../board-definition.schema.json",
    "name": "SKR v1.3",
    "manufacturer": "BIGTREETECH",
    "serialPath": "/dev/btt-skr-13",
    "firmwareBinaryName": "firmware-btt-skr-13.bin",
    "compileScript": "compile.sh",
    "flashScript": "flash.sh",
    "flashInstructions": "Flashing only works if there's a properly formatted (FAT16 or FAT32 with a small blocksize) micro SD card in the SKR Pro's SD Card slot. The smaller the SD card the better, a cheap 128mb TF-card is perfect.",
}