at91bootstrap: update PKG_MIRROR_HASH to zstd for v3 at91bootstrap

at91bootstrap: update PKG_MIRROR_HASH to zstd for v3 at91bootstrap

So, when updating the hash for at91bootstrap it was done via CHECK_ALL=1
so that updated the PKG_MIRROR_HASH for the main v4 version hash, but
at91bootstrap checkout version depends on the subtarget as well.

Choosing to build for sam9x will change the at91bootstrap version to v3
and this hash was not refreshed thus causing the CI to fail.

Fixes: 6918c637b797 ("treewide: package: update missed hashes after switch to ZSTD")
Signed-off-by: Robert Marko <robimarko@gmail.com>