sunxi-fel: find Homebrew libfdt on Darwin

Homebrew's dtc formula installs libfdt headers and libraries under
the dtc prefix. The existing sunxi-fel link line only passed -lfdt,
which leaves clang unable to find libfdt.h when the header is outside
the default include path.

Keep the default -lfdt behavior for systems that install libfdt in
default search paths. On Darwin, add the Homebrew dtc include and
library paths when that prefix is available.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

Author: jameshilliard

Makefile

@@ -134,6 +134,17 @@ ZLIB = zlib
 ZLIB_CFLAGS ?= `$(PKG_CONFIG) --cflags $(ZLIB)`
 ZLIB_LIBS ?= `$(PKG_CONFIG) --libs $(ZLIB)`
 
+ifeq ($(OS),Darwin)
+	DTC_PREFIX := $(shell brew --prefix dtc 2>/dev/null)
+	ifneq ($(DTC_PREFIX),)
+		LIBFDT_CFLAGS ?= -I$(DTC_PREFIX)/include
+		LIBFDT_LIBS ?= -L$(DTC_PREFIX)/lib -lfdt
+	endif
+endif
+
+LIBFDT_CFLAGS ?=
+LIBFDT_LIBS ?= -lfdt
+
 ifeq ($(OS),Windows_NT)
 	# Windows lacks mman.h / mmap()
 	DEFAULT_CFLAGS += -DNO_MMAP
@@ -149,8 +160,8 @@ FEL_LIB  := fel_lib.c fel_lib.h
 SPI_FLASH:= fel-spiflash.c fel-spiflash.h fel-remotefunc-spi-data-transfer.h
 
 sunxi-fel: fel.c fit_image.c thunks/fel-to-spl-thunk.h $(PROGRESS) $(SOC_INFO) $(FEL_LIB) $(SPI_FLASH)
-	$(CC) $(HOST_CFLAGS) $(LIBUSB_CFLAGS) $(ZLIB_CFLAGS) $(LDFLAGS) -o $@ \
-		$(filter %.c,$^) $(LIBS) $(LIBUSB_LIBS) $(ZLIB_LIBS) -lfdt
+	$(CC) $(HOST_CFLAGS) $(LIBUSB_CFLAGS) $(ZLIB_CFLAGS) $(LIBFDT_CFLAGS) $(LDFLAGS) -o $@ \
+		$(filter %.c,$^) $(LIBS) $(LIBUSB_LIBS) $(ZLIB_LIBS) $(LIBFDT_LIBS)
 
 sunxi-nand-part: nand-part-main.c nand-part.c nand-part-a10.h nand-part-a20.h
 	$(CC) $(HOST_CFLAGS) -c -o nand-part-main.o nand-part-main.c