uboot-zynq: fix build on hosts lacking pkg-config

The uboot-mvebu package incorrectly used the host pkg-config for the tool
build parts, which broke the build on systems lacking pkg-config and only
worked by accident on those that have it installed.

Export the host-build specific environment variables for the uboot build
to redirect pkg-config invocations to our staged host build pkg-config in
buildroot.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/boot/uboot-zynq/Makefile

@@ -14,6 +14,7 @@ PKG_HASH:=e49337262ecac44dbdeac140f2c6ebd1eba345e0162b0464172e7f05583ed7bb
 include $(INCLUDE_DIR)/kernel.mk
 include $(INCLUDE_DIR)/u-boot.mk
 include $(INCLUDE_DIR)/package.mk
+include $(INCLUDE_DIR)/host-build.mk
 
 define U-Boot/Default
   BUILD_TARGET:=zynq
@@ -42,4 +43,6 @@ UBOOT_TARGETS := \
 	zed \
 	zybo
 
+Build/Exports:=$(Host/Exports)
+
 $(eval $(call BuildPackage/U-Boot))