common/cmd_bootm.c: prevent running of subcommands before 'bootm start'

The execution order of the bootm subcommands is fixed.
Although here is a sanity check in the state machine
which should prevent running the subcommands in wrong
order but it does not catch all possible errors.

It is possible to run any subcommand without running
'bootm start' first which leads to unexpected behaviour.
For example, running 'bootm loados' without 'bootm start'
causes a hang:

    U-Boot> bootm loados
       XIP Invalid Image ... OK
    OK

Add a sanity check to 'do_bootm_subcommand' in order
to ensure that no subcommands can be executed before
'bootm start'.

After the patch running of any subcommand without running
'bootm start' will cause an error like this:

    U-Boot> bootm loados
    Trying to execute a command out of order
    bootm - boot application image from memory

    Usage:
    bootm [addr [arg ...]]
        - boot application image stored in memory
    ...

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>