scripts/unpack, scripts/extract: add support to extract '*.7z' files

Signed-off-by: Stephan Raue <stephan@openelec.tv>

scripts/extract

@@ -30,6 +30,10 @@ for s in `sed 's%.*/\(.*\)\$%\1%' $PKG_DIR/url`; do
       *.tar.gz | *.tgz)
         tar xzf $f -C $3
         ;;
+      *.7z)
+        mkdir -p $3/$1
+        7z x -o$3/$1 $f
+        ;;
       *.diff | *.patch)
         cat $f | patch -d $3 -p1
         ;;