bootconfig: Fix kernel message mentioning CONFIG_BOOT_CONFIG

Fix up one typo: CONFIG_BOOTCONFIG -> CONFIG_BOOT_CONFIG

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>