sdk: set package as the root directory for base feed

sdk: set package as the root directory for base feed

To produce consistent source entry for package compiled from non-SDK and
SDK build, set the "--root=package" flag for the base feed.

This will set the root directory for the base feed to the OpenWrt
core repository "package" directory.

This fix the reproducible problem of package build from SDK that have
the source entry set to "feeds/base/package/..." for every package
coming from the base feed.

Link: https://github.com/openwrt/openwrt/pull/20396
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>