bootm: Fix flags used for bootargs string substitution

Commit 51bb3384 ("bootm: Support string substitution in
bootargs") introduced a feature of bootargs string substitution
and changed a flag used in bootm_process_cmdline_env() call
to be either true or false. With this flag value,
condition in bootm_process_cmdline()
`if (flags & BOOTM_CL_SUBST)` is never true
and process_subst() is never called.

Add a simple test to verify if substitution works OK.

Signed-off-by: Piotr Kubik <piotr.kubik@iopsys.eu>
Reviewed-by: Simon Glass <sjg@chromium.org>