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Description
By simplifying the configuration of the load/tftp plugin to only a --load-tftp-dir argument and recommending one blessed TFTP server, we reduce the things that possibly go wrong, and would be able to simplify the implementation of several boot_dir plugins (like https://github.com/genodelabs/genode/blob/master/tool/run/boot_dir/foc#L195).
Right now, the code that concatenates the base and offset dirs is complicated, contains many duplicated, and cannot use Tcl's file join function because of the leading / of the offset dir (see http://www.tcl-lang.org/man/tcl/TclCmd/file.htm#M19). Hence, it calls for being cleaned up.