P2.8.5.7

• Firmware - Disabled firmware downgrade on 3B+
• Clear - Added Clear button to clear all selections
• Kernel Bump - Linux kernel 4.14.74, firmware and userland bumped to match NOOBS 2.9
• Replace - Removed RiscOS & Windows_10 from OS replacement
• SilentInstall - Fixed waiting for USB images
• networktimeout - 0 means wait indefinitely for network before silentinstall