[kernel] add extra tag support (infrastructure for kernel downloading)

[yxieca]

Extra tag support is introduced to solve following issues:

1. ensure that with any checked out code, When we can successfully build a kernel, and successfully download one, they are the same.
2. when we add a patch to an existing kernel source code package, we can use extra tag to distinguish them.

When download fails, one can always build one.

[lguohan]

is this still needed?

[jarias-lfx]

/easycla