build: Add support of generating kernel includes for prebuilt kernels

For prebuilt kernels, point TARGET_KERNEL_HEADERS_SOURCE to the dummy
kernel source used to generate headers for HALs.

Change-Id: I95a1050a7e0650faac112724b7bd6a300cdb6e4a

build/soong/Android.bp

@@ -21,7 +21,7 @@ twrp_generator {
     name: "generated_kernel_includes",
 
     // The headers make command
-    cmd: "$(PATH_OVERRIDE_SOONG) $(KERNEL_MAKE_CMD) $(KERNEL_MAKE_FLAGS) -C $(TARGET_KERNEL_SOURCE) O=$(KERNEL_BUILD_OUT_PREFIX)$(genDir) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) headers_install",
+    cmd: "$(PATH_OVERRIDE_SOONG) $(KERNEL_MAKE_CMD) $(KERNEL_MAKE_FLAGS) -C $(TARGET_KERNEL_HEADERS_SOURCE) O=$(KERNEL_BUILD_OUT_PREFIX)$(genDir) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) headers_install",
 
     // Directories that can be imported by a cc_* module generated_headers property
     export_include_dirs: [
@@ -32,7 +32,7 @@ twrp_generator {
     ],
 
     // Sources for dependency tracking
-    dep_root: "$(TARGET_KERNEL_SOURCE)",
+    dep_root: "$(TARGET_KERNEL_HEADERS_SOURCE)",
     dep_files: [
         "Makefile",
         "include/**/*",

config/BoardConfigKernel.mk

@@ -20,6 +20,9 @@
 #
 #   TARGET_KERNEL_SOURCE               = Kernel source dir, optional, defaults
 #                                          to kernel/$(TARGET_DEVICE_DIR)
+#   TARGET_KERNEL_HEADERS_SOURCE       = Kernel headers dir, in order to generate kernel
+#                                          includes for prebuilt kernels, optional, defaults
+#                                          to TARGET_KERNEL_SOURCE
 #   TARGET_KERNEL_ADDITIONAL_FLAGS     = Additional make flags, optional
 #   TARGET_KERNEL_ARCH                 = Kernel Arch
 #   TARGET_KERNEL_CROSS_COMPILE_PREFIX = Compiler prefix (e.g. arm-eabi-)
@@ -45,6 +48,10 @@ ifneq ($(TARGET_PREBUILT_KERNEL),)
 TARGET_KERNEL_SOURCE :=
 endif
 
+ifneq ($(TARGET_KERNEL_SOURCE),)
+TARGET_KERNEL_HEADERS_SOURCE := $(TARGET_KERNEL_SOURCE)
+endif
+
 TARGET_KERNEL_ARCH := $(strip $(TARGET_KERNEL_ARCH))
 ifeq ($(TARGET_KERNEL_ARCH),)
 KERNEL_ARCH := $(TARGET_ARCH)

config/BoardConfigSoong.mk

@@ -9,7 +9,7 @@ EXPORT_TO_SOONG := \
     KERNEL_MAKE_FLAGS \
     PATH_OVERRIDE_SOONG \
     TARGET_KERNEL_CONFIG \
-    TARGET_KERNEL_SOURCE
+    TARGET_KERNEL_HEADERS_SOURCE
 
 # Setup SOONG_CONFIG_* vars to export the vars listed above.
 # Documentation here: