[FS#1529] Preserve oem firmware in spare partition

[LEDE Bugs]

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FS#1529 - Preserve oem firmware in spare partition
User who did this: Yousong Zhou (yousong)
Summary: Feature request: preserve official firmware in spare partition -> Preserve oem firmware in spare partition

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=1529