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

u-boot 2016.05 no longer builds with the default gcc/binutils versions:

https://gitlab.com/buildroot.org/buildroot/-/jobs/88314946



  LD      u-boot
fs/built-in.o: In function `read_symbol':
/builds/buildroot.org/buildroot/output/build/uboot-2016.05/fs/jffs2/mini_inflate.c:129: undefined reference to `pull_bit'
fs/built-in.o: In function `decompress_huffman':
/builds/buildroot.org/buildroot/output/build/uboot-2016.05/fs/jffs2/mini_inflate.c:159: undefined reference to `pull_bits'
/builds/buildroot.org/buildroot/output/build/uboot-2016.05/fs/jffs2/mini_inflate.c:170: undefined reference to `pull_bits'
..
/builds/buildroot.org/buildroot/output/host/bin/arm-buildroot-linux-uclibcgnueabi-ld.bfd: BFD (GNU Binutils) 2.29.1 assertion fail elf32-arm.c:9509
Makefile:1192: recipe for target 'u-boot' failed
make[1]: *** [u-boot] Error 1
make[1]: Leaving directory '/builds/buildroot.org/buildroot/output/build/uboot-2016.05'

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