build: move install dirs out of the build dir

Instead of cluttering $BUILD we now have
- $BUILD/build         sources and builds of packages
- $BUILD/install_pkg   installed packages
- $BUILD/install_init  installed packages for initramfs

scripts/build

@@ -485,6 +485,10 @@ if [ "${TARGET}" = "target" -o "${TARGET}" = "init" ]; then
   fi
 fi
 
+if [ -n "${INSTALL}" -a -d "${INSTALL}" ]; then
+  echo "INFO_PKG_NAME=\"${PKG_NAME}\"" > "${INSTALL}/.libreelec-package"
+fi
+
 cd ${ROOT}
 
 PKG_DEEPHASH=$(calculate_stamp)