distro: not taint environment variables if possible

The aim of this patch is not to have temporary variables used in
distro_bootcmd left as environment variables after run something.
See the discussion[1].
Without this patch, saveenv command also saves those variables, too.
While they are apparently safe, scsi_need_init can be harmful.

Please note that, in most cases, a variable should be converted to
hush's local variable, while "devplist" cannot because it is created
by "part" command as an environment variable.

[1] https://lists.denx.de/pipermail/u-boot/2018-December/350209.html



Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>