fsl-layerscape/soc.c: do not destroy bootcmd environment

When an XXX_BOOTCOMMAND isn't defined, the result is that bootcmd is set
to some random memory content. Fix it so that the function does nothing
in that case and leaves the bootcmd environment unmodified.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>