uboot: use local libfdt.h

Use of libfdt.h by u-boot is currently using
the <system-path>.

Jan submitted the following upstream
https://patchwork.ozlabs.org/patch/833760/

Resolves a failure like this one observed on uboot-tools
http://autobuild.buildroot.net/results/347cde4b5c0e6ca76d354396385be4ec1294da73

[Peter: only sed if file exists]
CC: Jan Kundrát <jan.kundrat@cesnet.cz>
CC: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/uboot.mk

@@ -183,6 +183,13 @@ define UBOOT_APPLY_LOCAL_PATCHES
 endef
 UBOOT_POST_PATCH_HOOKS += UBOOT_APPLY_LOCAL_PATCHES
 
+# Bug: https://patchwork.ozlabs.org/patch/833760/
+define UBOOT_FIX_LIBFDT_SYSTEM_PATH
+	[ ! -e $(@D)/tools/fdtgrep.c ] || \
+	$(SED) 's%<../include/libfdt.h>%"../include/libfdt.h"%' $(@D)/tools/fdtgrep.c
+endef
+UBOOT_POST_PATCH_HOOKS += UBOOT_FIX_LIBFDT_SYSTEM_PATH
+
 ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY),y)
 define UBOOT_CONFIGURE_CMDS
 	$(TARGET_CONFIGURE_OPTS) 	\