sandbox: capsule: Add path to the public key ESL file

Add the path to the public key EFI Signature List(ESL) file for the
sandbox variants which enable capsule authentication. This ESL file
gets embedded into the platform's device-tree as part of the build.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index c20015d..db916e6 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -226,6 +226,7 @@
 CONFIG_EFI_CAPSULE_ON_DISK=y
 CONFIG_EFI_CAPSULE_FIRMWARE_FIT=y
 CONFIG_EFI_CAPSULE_AUTHENTICATE=y
+CONFIG_EFI_CAPSULE_ESL_FILE="board/sandbox/capsule_pub_esl_good.esl"
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y