imx8m*_venice_defconfig: fix default bootcmd

commit 970bf860 ("Convert CONFIG_USE_BOOTCOMMAND et al to Kconfig")
had an unintended side effect of resulting in a bootcmd env var change
for boards like venice that did not have CONFIG_USE_BOOTCOMMAND defined
and relied on it being defaulted in include/config_distro_bootcmd.h.
Following that patch it instead got defaulted in tools/env/fw_env_private.h

Fix this by enabling CONFIG_USE_BOOTCOMMAND for venice.

Fixes: commit 970bf860 ("Convert CONFIG_USE_BOOTCOMMAND et al to Kconfig")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>