Add build variant to kernel command line

Verity is not enabled in eng builds. Pass the build variant so
that kernel does not try to enable verity in eng builds

BUG:29276559
Change-Id: I7f412196ac59aa63e91c21d825ad15bae9f51691
diff --git a/core/Makefile b/core/Makefile
index 79faae1..cdf24dd 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -550,7 +550,7 @@
 endif
 endif
 
-BOARD_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE) $(VERITY_KEYID))
+BOARD_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE) buildvariant=$(TARGET_BUILD_VARIANT) $(VERITY_KEYID))
 ifdef BOARD_KERNEL_CMDLINE
 INTERNAL_BOOTIMAGE_ARGS += --cmdline "$(BOARD_KERNEL_CMDLINE)"
 endif