tools/bootconfig: Add testcases for value override operator

Add some testcases and examples for value override operator.

Link: https://lkml.kernel.org/r/159482883824.126704.2166030493721357163.stgit@devnote2



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>