autodetect.go: combine tow switch cases
autodetect.go: combine tow switch cases

Signed-off-by: Ling FaKe <25fffa53a086899a13ee0dcbe820593f9964b232@huawei.com>

xiekeyang/image-tools,xiekeyang/image-tools,opencontainers/image-tools,opencontainers/image-tools