cmd: mvebu/bubt: Add support for sha512 checksum validation for Armada 3700

Armada 3700 BootROM supports also images with sha512 checksums and
mox-imager tool [1] generates such bootable images. Without sha512 support
U-Boot bubt command just prints error message:

  Error: Unsupported hash_algorithm_id = 64
  Error: Image header verification failed

This patch adds support for sha512 checksum validation for Armada 3700
images. With it bubt prints:

  Image checksum...OK!

[1] - https://gitlab.nic.cz/turris/mox-boot-builder.git

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/cmd/mvebu/Kconfig b/cmd/mvebu/Kconfig
index 39963db..120397d 100644
--- a/cmd/mvebu/Kconfig
+++ b/cmd/mvebu/Kconfig
@@ -4,6 +4,7 @@
 config CMD_MVEBU_BUBT
 	bool "bubt"
 	select SHA256 if ARMADA_3700
+	select SHA512 if ARMADA_3700
 	help
 	  bubt - Burn a u-boot image to flash
 	  For details about bubt command please see the documentation