build: Common exFAT KM build target for reuse by devices

Devices can still use their own build targets, or can simply
include the exfat-km.mk herein into their BoardConfig
(or call inherit-product in other device MK).

Compatible with the inline kernel building in CM13/4, and similar.

Change-Id: Ib50654d2925ac26233e1c984e079990b70383dae

exfat-km.mk

+EXFAT_FOLDER ?= external/exfat-nofuse
+
+EXFAT_MODULE:
+	make clean -C $(EXFAT_FOLDER) KDIR=$(KERNEL_OUT)
+	make -j8 -C $(EXFAT_FOLDER) ARCH=arm KDIR=$(KERNEL_OUT) \
+		$(if $(ARM_CROSS_COMPILE),$(ARM_CROSS_COMPILE),$(KERNEL_CROSS_COMPILE))
+	mv $(EXFAT_FOLDER)/exfat.ko $(KERNEL_MODULES_OUT)
+	$(if $(ARM_EABI_TOOLCHAIN),$(ARM_EABI_TOOLCHAIN)/arm-eabi-strip, \
+		$(KERNEL_TOOLCHAIN_PATH)strip) --strip-unneeded $(KERNEL_MODULES_OUT)/exfat.ko
+
+TARGET_KERNEL_MODULES += EXFAT_MODULES