Makefile: Fix UEFI devkey discovery

Include UEFI key directories (db/, kek/, pk/) in devkeys_install
target. Without this change, scripts/image_signing/sign_uefi.sh
aborted, complaining about missing UEFI keys.

BUG=b/465211305
TEST=presubmit
RELEASE_NOTE=None

cos-patch: bug
Change-Id: I4b6d1473eae831a7ba697fb69b4c0cffffdb971b
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/152103
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/154334

Makefile

diff --git a/Makefile b/Makefile
index 7731598..a24170b 100644
--- a/Makefile
+++ b/Makefile
@@ -964,6 +964,7 @@
 	${Q}mkdir -p ${US_DIR}/devkeys
 	${Q}${INSTALL} -t ${US_DIR}/devkeys -m644 \
 		`find tests/devkeys -type f -maxdepth 1`
+	${Q}cp -r tests/devkeys/uefi/* ${US_DIR}/devkeys/
 
 # ----------------------------------------------------------------------------
 # CGPT library and utility