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 <hauke@hauke-m.de>
(cherry picked from commit a141e7a00e3ad8442831ed87766451a6114afdf9)

package/boot/at91bootstrap/Makefile

@@ -14,6 +14,7 @@ PKG_RELEASE:=
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://github.com/linux4sam/at91bootstrap.git
+PKG_MIRROR_HASH:=783100ce586460b8230f7b5b56cbc2e6473b888f38330c1d1b64777e3ae59473
 PKG_SOURCE_VERSION:=04efa5500d60a0211d14b6ee60df7ce0a828704d
 
 PKG_BUILD_DIR = \