bootconfig: Prohibit re-defining value on same key

Currently, bootconfig adds a new value on the existing key to the tail of an
array. But this looks a bit confusing because an admin can easily rewrite
the original value in the same config file.

This rejects the following value re-definition.

  key = value1
  ...
  key = value2

You should rewrite value1 to value2 in this case.

Link: http://lkml.kernel.org/r/158227282199.12842.10110929876059658601.stgit@devnote2

Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
[ Fixed spelling of arraies to arrays ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tools/bootconfig/samples/bad-samekey.bconf

@@ -0,0 +1,6 @@
+# Same key value is not allowed
+key {
+	foo = value
+	bar = value2
+}
+key.foo = value