x86: Rewrite bootm.c to make it similar to ARM

The x86 bootm code is quite special, and geared to zimage. Adjust it
to support device tree and make it more like the ARM code, with
separate bootm stages and functions for each stage.

Create a function announce_and_cleanup() to handle printing the
"Starting kernel ..." message and put it in bootm so it is in one
place and can be used by any loading code. Also move the
board_final_cleanup() function into bootm.

Signed-off-by: Simon Glass <sjg@chromium.org>