Refactor the bootm command to reduce code duplication

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>