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/board_config.mk b/core/board_config.mk
index fecb6f6..1e0e9e1 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -82,6 +82,7 @@
 BOARD_KERNEL_BASE := $(strip $(BOARD_KERNEL_BASE))
 BOARD_KERNEL_PAGESIZE := $(strip $(BOARD_KERNEL_PAGESIZE))
 
+INTERNAL_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE))
 
 ifeq ($(TARGET_CPU_ABI),)
   $(error No TARGET_CPU_ABI defined by board config: $(board_config_mk))