Add GKI global variable.

Add BOARD_USES_GENERIC_KERNEL_IMAGE to be the global variable to
indicate the device uses GKI image.

Test: pass

Bug: 156098440

Change-Id: Ica553bbdb791e25dfa9bafb524ab2de5e3f8720d
diff --git a/core/board_config.mk b/core/board_config.mk
index 05b6b29..8736505 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -97,6 +97,13 @@
   BOARD_KERNEL_BINARIES \
   BOARD_KERNEL_MODULE_INTERFACE_VERSIONS \
 
+# Variables related to generic kernel image (GKI) and generic boot image
+# - BOARD_USES_GENERIC_KERNEL_IMAGE is the global variable that defines if the
+#   board uses GKI and generic boot image.
+#   Update mechanism of the boot image is not enforced by this variable.
+_board_strip_readonly_list += \
+  BOARD_USES_GENERIC_KERNEL_IMAGE \
+
 _build_broken_var_list := \
   BUILD_BROKEN_DUP_RULES \
   BUILD_BROKEN_DUP_SYSPROP \