at91bootstrap: Add PKG_MIRROR_HASH to fix download

The referenced commit is gone, but we already have this file on our
mirror, use that one by providing the correct mirror hash.

I generated a tar.xz file with the given git commit hash using a random
fork on github and it generated the same tar.xz file as found on our
mirror so this looks correct.

Signed-off-by: Hauke Mehrtens <[email protected]>

package/boot/at91bootstrap/Makefile

@@ -14,6 +14,7 @@ PKG_RELEASE:=2
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://github.com/linux4sam/at91bootstrap.git
+PKG_MIRROR_HASH:=06753d673756edc9753932db00f4e5b8c1f9fa7708337c4d6ce280573efb86b4
 PKG_SOURCE_VERSION:=d96833a4b6680b237708eb4dc9f10708b9e709d8
 PKG_BUILD_DIR = \
 	$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_VERSION)