build: use mkhash to replace various quirky md5sum/openssl calls

Signed-off-by: Felix Fietkau <nbd@nbd.name>

scripts/combined-ext-image.sh

@@ -47,7 +47,7 @@ while [ "$#" -gt 1 ]
 
       [ ! -f "$file" ] && echo "$ME: Not a valid file: $file" && usage
       FILES="$FILES $file"
-      md5=$(cat "$file" | md5sum -)
+      md5=$(mkhash md5 "$file")
       printf "%-32s%08x%32s" "$filename" $(stat -c "%s" "$file") "${md5%% *}" >> $IMG_OUT
       shift 2
    done