Add bootconfig to kernel cmdline when BOARD_BOOTCONFIG not empty

The 'bootconfig' kernel cmdline parameter needs to exist for the kernel
to search for bootconfig.
If BOARD_BOOTCONFIG contains anything, then add the parameter to the
kernel cmdline.

Test: Boot cuttlefish and verify /proc/cmdline has 'bootconfig' after
removing it from cuttlefish BoardConfig.mk
Bug: 173815685

Change-Id: I112a2a8e02ba7265c5547d9244298e07f26985ba
diff --git a/core/board_config.mk b/core/board_config.mk
index 245a639..57363fb 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -223,7 +223,10 @@
 .KATI_READONLY := $(_board_strip_readonly_list)
 
 INTERNAL_KERNEL_CMDLINE := $(BOARD_KERNEL_CMDLINE)
-INTERNAL_BOOTCONFIG := $(BOARD_BOOTCONFIG)
+ifneq (,$(BOARD_BOOTCONFIG))
+  INTERNAL_KERNEL_CMDLINE += bootconfig
+  INTERNAL_BOOTCONFIG := $(BOARD_BOOTCONFIG)
+endif
 
 ifneq ($(filter %64,$(TARGET_ARCH)),)
   TARGET_IS_64_BIT := true