sys/tools: Add a tool for generating arm and arm64 kernel images.

This tool can generate kernel images without changing the offsets in
the final executable. It replaces the ELF header by properly sized zeroed
block then emits a relative jump to _start(for 'v7jump' or 'v8jump' option)
or the booti header (for 'v8booti' option) to the beginning of the converted file.
Submited by: ian