RPI3B+
Added RPI_MODEL=3B for Raspberry Pi 3 B+ Model
If RPI_MODEL=3B is set rpi23-gen-image will use brcmfmac43455-sdio (bin, txt and clm_blob) instead of brcmfmac43430-sdio WLAN-firmware blob
With Model=3 - rpi23-gen-image.sh will not copy brcmfmac43455-sdio.txt and brcmfmac43455-sdio.clm_blob file from firmware_dir. Package firmware-brcm80211 also does not include these files.