juno.conf: use arch-armv8a tune instead of arch-armv8
juno.conf: use arch-armv8a tune instead of arch-armv8

arch-armv8 was removed from latest OE-Core.

Signed-off-by: Ricardo Salveti <d4e7430f1534a12df46cedd1ac369935436dbb94@foundries.io>

96boards/meta-96boards,mrchapp/meta-96boards