omni: Allow to opt out from omni kernel build

GKI devices should build the kernel within the hermetic kernel build environment.
https://source.android.com/docs/core/architecture/kernel/stable-kmi#hermetic-build-process

Change-Id: Id920c433a9b68b77cdee1cdd29ae1ae8309aed2c
diff --git a/config/BoardConfigOmni.mk b/config/BoardConfigOmni.mk
index a421d1d..5a47ce5 100644
--- a/config/BoardConfigOmni.mk
+++ b/config/BoardConfigOmni.mk
@@ -1,2 +1,5 @@
+ifneq ($(TARGET_USES_KERNEL_PLATFORM),true)
 include vendor/omni/config/BoardConfigKernel.mk
+endif
+
 include vendor/omni/config/BoardConfigSoong.mk