Refactor the bootm command to reduce code duplication

At present the bootm code is mostly duplicated for the plain 'bootm'
command and its sub-command variant. This makes the code harder to
maintain and means that changes must be made to several places.

Introduce do_bootm_states() which performs selected portions of the bootm
work, so that both plain 'bootm' and 'bootm <sub_command>' can use the
same code.

Additional duplication exists in bootz, so tidy that up as well. This
is not intended to change behaviour, apart from minor fixes where the
previously-duplicated code missed some chunks of code.

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