Modify INTERNAL_KERNEL_CMDLINE instead of BOARD_ variant

The current modification of the variable within the build system
prohibits making the variable readonly.

Test: build_test
Test: boot blueline
Change-Id: Iec6058a163b5951db389d4adc0d9d0397431efb1
diff --git a/core/Makefile b/core/Makefile
index 3cc9831..9a2d985 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -930,7 +930,7 @@
 endif
 endif
 
-INTERNAL_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE) buildvariant=$(TARGET_BUILD_VARIANT) $(VERITY_KEYID))
+INTERNAL_KERNEL_CMDLINE := $(strip $(INTERNAL_KERNEL_CMDLINE) buildvariant=$(TARGET_BUILD_VARIANT) $(VERITY_KEYID))
 ifdef INTERNAL_KERNEL_CMDLINE
 INTERNAL_BOOTIMAGE_ARGS += --cmdline "$(INTERNAL_KERNEL_CMDLINE)"
 endif
@@ -1021,7 +1021,6 @@
 endif
 $(eval $(call copy-one-file,$(BOARD_PREBUILT_BOOTIMAGE),$(INSTALLED_BOOTIMAGE_TARGET)))
 else # BOARD_PREBUILT_BOOTIMAGE not defined
-INTERNAL_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE))
 INSTALLED_BOOTIMAGE_TARGET :=
 endif # BOARD_PREBUILT_BOOTIMAGE
 endif # TARGET_NO_KERNEL