Documentation: bootconfig: Add bootconfig override operator

Add a sentence about bootconfig override operator (":=") to
bootconfig.rst.

Link: https://lkml.kernel.org/r/159482884682.126704.7198860675721719878.stgit@devnote2



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