arm64: boot-wrapper: add mirror checksum

Some builders have troubles cloning the boot-wrapper-aarch64.git repository so
provide a mirror checksum to allow downloading from sources.lede-project.org
instead.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/arm64/image/boot-wrapper/Makefile

@@ -15,6 +15,7 @@ PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://git.kernel.org/pub/scm/linux/kernel/git/mark/boot-wrapper-aarch64.git
 PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_VERSION)
 PKG_SOURCE_VERSION:=c51dde817b5ed5b8f741b67ac51bd67bd87b4a2a
+PKG_MIRROR_MD5SUM:=2f029a20c906af68e9fe067c1b0ed394e4c455e142454a039f41d1d82d6e17c8
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION)-$(PKG_SOURCE_VERSION).tar.gz
 PKG_MAINTAINER:=Florian Fainelli <florian@openwrt.org>