boot/at91bootstrap3: fix hash

boot/at91bootstrap3: fix hash

When at91bootstrap3 was bumped to 3.9.0 in commit
513899e471890f4eb677066876890d43ea91b25e, an incorrect hash was set:
it was the hash of the tarball generated by the Git download logic of
Buildroot, and not the hash of the tarball auto-generated by Github,
which is the one that we really download by default.

Fixes:

ERROR: at91bootstrap3-v3.9.0.tar.gz has wrong sha256 hash:
ERROR: expected: 9960b0d18fe42feee566d4c52efa0d7c8251685bf9acfdf343f30a27951ada1e
ERROR: got     : e23e6df23b79ca81e412cb73a1f48bd95df8d46c7d52a1d073c2ed9d4f3a1a71
ERROR: Incomplete download, or man-in-the-middle (MITM) attack

Reported-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
[Thomas: improved commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>