add a new args for nydus-image use to generate aligned blobcache file

[zyfjeff]

Details about issue

At present, the nydus blob we generate is not directly used by nydusd, but first downloaded, decompressed and finally written to the blobcache file, if we can directly generate the blobcache file when the image is built, nydusd directly uses this file to start. The process of writing data from Nydus blob copy data to blobcache files is avoided. This is an optimization for localfs mode.