kdb: Refactor env variables get/set code

Add two new kdb environment access methods as kdb_setenv() and
kdb_printenv() in order to abstract out environment access code
from kdb command functions.

Also, replace (char *)0 with NULL as an initializer for environment
variables array.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/1612771342-16883-1-git-send-email-sumit.garg@linaro.org
[daniel.thompson@linaro.org: Replaced (char *)0/NULL initializers with
an array size]
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>