rpi: update to 2018.07 U-boot

This is to sync up with the U-boot version that is in the latest
Yocto release. This will also resolve the requirement to run
gcc6, with 2018.07 we can build with gcc7 which is the default
in most distributions.

MEN-2198

Changelog: Title

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>

rpi-convert-stage-5.sh

@@ -38,8 +38,8 @@ version() {
 build_uboot_files() {
   local CROSS_COMPILE=${1}-
   local ARCH=arm
-  local branch="mender-rpi-2017.09"
-  local commit="9214bb597e"
+  local branch="mender-rpi-2018.07"
+  local commit="981cc831e3"
   local uboot_repo_vc_dir=$uboot_dir/.git
   local defconfig="rpi_3_32b_defconfig"