多个metaimage的烧写地址，是rom代码里支持当第一个meta image没有找到（例如flash内容损坏），可以跳到后面的备份image，仍然可以正常工作。
在AWR1642 bootloader flow文档（SWRA551）里可以看到下面的信息：
Fallback images: the bootloader supports loading of images from the following locations as a fallback
mechanism if one of the images is corrupted in the SDF. The locations of the images are:
– META IMG1(SDF offset – 0x0)
– META IMG2(SDF offset – 0x80000)
– META IMG3(SDF offset – 0x100000)
– META IMG4(SDF offset – 0x180000)
