bootm: Add the missing PREP stage to bootz and correct image handling

In the recent bootm refactor, the PREP stage was missing in the bootz
command. This causes unpredictable behaviour.

The use of a local variable means that the reset of cmd_bootm.c does not
in fact use the same image structure, so remove this.

Also manually set the OS type to Linux, since this is the only possibility
at present, and we need to select the right boot function.

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