Tidy up and document testing.

	* dejagnu/baseboards/atmega128-studio.exp: Deleted.
	* dejagnu/baseboards/atmel-studio.exp: Improved way of specifying
	use of installed compiler.
	* dejagnu/gdb-extra.exp: Note usage in log file.
	(gdb_load): Verbose reporting of remote timeout.
	* dejagnu/gdb-stub-extra.exp: Note usage in log file.
	* README.md: Updated to document testing.