cli: Export cli_simple_process_macros for use outside of cli_simple

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
pxe: Allow use of environment variables in append string

Use cli_simple_process_macros, so that environment
variables (e.g. ${console}) can be used in append strings.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

--
Changes in v2:
-Use cli_simple_process_macros instead of run_cmd("setenv bootargs ..."), so
 that only $() / ${} get processed and not ;