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>