Revert some commits for performance degration in CPU DUST.

Revert "separate compile the kernel"

This reverts commit a2b2146b9cb7f1df858ea879ab106e3861cc9dfc.

Revert "DUST use dust_inner instead of ckernel"

This reverts commit 6b12a6990b9ca9db0a5180b8804173772fb8cfd9.

Revert "add dust_inner"

This reverts commit 0960f7e7f8e020c94c1a3e4dbec56f7824c2a0c4.