base-files: coreutil-sha256sum breaks status code

With package "coreutil-sha256sum" installed "sysupgrade" fails to perform 'sha256sum -s' and instead returns 'invalid option -- 's''.
This is caused due to:
	different syntax for a sha256sum status check ('sha256sum --status' with "coreutil-sha256sum")
	'/usr/bin/sha256sum' being symlinked to '/usr/bin/gnu-sha256sum' (after installation of "coreutil-sha256sum")
"coreutil-sha256sum" package from the packages feed replaces the Busybox sha256sum
This patch restores for 'sysupgrade' the busybox call to its sha256sum applet.

Signed-off-by: Huangbin Zhan <zhanhb88@gmail.com>

package/base-files/files/sbin/sysupgrade

@@ -127,7 +127,7 @@ list_changed_conffiles() {
 	list_conffiles | while read file csum; do
 		[ -r "$file" ] || continue
 
-		echo "${csum}  ${file}" | sha256sum -sc - || echo "$file"
+		echo "${csum}  ${file}" | busybox sha256sum -sc - || echo "$file"
 	done
 }