buildroot

pandaboard_defconfig: bump u-boot to 2017.05 to fix build with gcc 6

The recent change to default to gcc 6 for the internal toolchain broke this
defconfig as the u-boot doesn't contain commit 9b2c282b34 (compiler*.h: sync
include/linux/compiler*.h with Linux 4.5-rc6) which was added during the
2016.03 cycle.

Fix the build by bumping u-boot to 2017.05.

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