sandbox: Enable HEXDUMP for sandbox_flattree

At present the hexdump tests are disabled in sandbox_flattree. This is
good, because they do not pass. Enable the required Kconfig so that they
will, when enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 7cc76bf..f184723 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -203,6 +203,7 @@
 CONFIG_TPM=y
 CONFIG_LZ4=y
 CONFIG_ERRNO_STR=y
+CONFIG_HEXDUMP=y
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y