bootconfig: Allocate xbc_data inside xbc_init()

Allocate 'xbc_data' in the xbc_init() so that it does
not need to care about the ownership of the copied
data.

Link: https://lkml.kernel.org/r/163177339986.682366.898762699429769117.stgit@devnote2

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