test: enable RSA library test on sandbox

We want to always run RSA library test on sandbox build in Travis CI.
Just adding CONFIG_RSA_VERIFY_WITH_PKEY would be good enough for this
purpose.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index cb387e7..53c5bd8 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -198,6 +198,7 @@
 CONFIG_FS_CRAMFS=y
 # CONFIG_SPL_USE_TINY_PRINTF is not set
 CONFIG_CMD_DHRYSTONE=y
+CONFIG_RSA_VERIFY_WITH_PKEY=y
 CONFIG_TPM=y
 CONFIG_LZ4=y
 CONFIG_ERRNO_STR=y