usb-boot: Remove prebuilt fel boot binaries

They were all built with no version information so it's not trivial
to tell which sources they are built from.

It is very easy to rebuild u-boot which gives these. Not really a need
to have them prebuilt here.
usb-boot: Improve messages somewhat explainign that it wants u-boot SPL FEL
usb-boot: ramboot script without uimage header
usb-boot: Check number of arguments before eating them