FROMLIST: bootconfig: Change array value to use child node

It is not possible to put an array value with subkeys under
a key node, because both of subkeys and the array elements
are using "next" field of the xbc_node.

Thus this changes the array values to use "child" field in
the array case. The reason why split this change is to
test it easily.

Picking this up FROMLIST to get the fix ASAP. It is likely to be
accepted upstream.

Signed-off-by: Devin Moore <devinmoore@google.com>
Link: https://lore.kernel.org/lkml/162262192121.264090.6540508908529705156.stgit@devnote2/


Bug: 183237066
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Change-Id: Iabd54cda32f6bde12297942d6b03bd2555e6eb0c