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_defconfig b/configs/sandbox_defconfig
index a57ab23..0f01471 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -340,6 +340,7 @@
 CONFIG_EFI_CAPSULE_ON_DISK=y
 CONFIG_EFI_CAPSULE_FIRMWARE_RAW=y
 CONFIG_EFI_CAPSULE_AUTHENTICATE=y
+CONFIG_EFI_CAPSULE_ESL_FILE="board/sandbox/capsule_pub_esl_good.esl"
 CONFIG_EFI_SECURE_BOOT=y
 CONFIG_TEST_FDTDEC=y
 CONFIG_UNIT_TEST=y