orangepi_pc_defconfig: bump u-boot to 2018.07 to fix build issue

u-boot 2018.05 now fails to build with the following error:

  HOSTCC  scripts/dtc/flattree.o
In file included from /builds/buildroot.org/buildroot/output/host/include/libfdt.h:54:0,
                 from /builds/buildroot.org/buildroot/output/build/uboot-2018.05/scripts/dtc/libfdt/fdt.c:54:
/builds/buildroot.org/buildroot/output/host/include/libfdt_env.h:82:24: error: redefinition of 'fdt16_to_cpu'
 static inline uint16_t fdt16_to_cpu(fdt16_t x)
                        ^~~~~~~~~~~~
In file included from /builds/buildroot.org/buildroot/output/build/uboot-2018.05/scripts/dtc/libfdt/fdt.c:51:0:
/builds/buildroot.org/buildroot/output/build/uboot-2018.05/scripts/dtc/libfdt/libfdt_env.h:81:24: note: previous definition of 'fdt16_to_cpu' was here
 static inline uint16_t fdt16_to_cpu(fdt16_t x)
                        ^~~~~~~~~~~~

https://gitlab.com/buildroot.org/buildroot/-/jobs/88314886

Fix it by bumping the u-boot version to 2018.07.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/orangepi_pc_defconfig

@@ -20,7 +20,7 @@ BR2_TARGET_ROOTFS_EXT2_4=y
 BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION=y
-BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2018.05"
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2018.07"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="orangepi_pc"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
 BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y