bootm: Create bootm_final() for pre-boot cleanup

bootm: Create bootm_final() for pre-boot cleanup

There are various functions which announce that booting is imminent and
do related preparation. Most of these are arch-specific.

In practice, most archs do a similar thing. It would be better to
have a common function, with perhaps some events for things that are
really arch- and board-specific.

Create a new bootm_final() function with the common pre-boot steps:
printing the "Starting kernel" message, recording bootstage data,
optionally writing bootstage to the FDT and printing a report, and
removing active devices.

Be careful to avoid using BIT() macros which are not available with host
tools.

Signed-off-by: Simon Glass <simon.glass@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>