image/populate_sdk: seprate variables to fix dependency

image/populate_sdk: seprate variables to fix dependency

Previously, do_rootfs depends on variables like SDK_OS, SDK_OUTPUT, etc.
And changing variables like POPULATE_SDK_POST_HOST_COMMAND doesn't cause
do_populate_sdk to rerun.

This patch separates variables so that do_rootfs and do_populate_sdk could
correctly depend on their related variables.

[YOCTO #8670]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>