The meta-schulung layer does not contain anything relating to u-boot.
Following is information for building/running u-boot with the vexpress
board.

(specify u-boot target)
echo 'UBOOT_MACHINE = "vexpress_ca9x4_defconfig"' >> conf/machine/vexpress.conf

(build u-boot)
bitbake u-boot

(wrap u-boot in uimage)
B='tmp/work/vexpress-schulung-linux-gnueabi/u-boot/1_2018.07-r0/build'
O='tmp/deploy/images/vexpress'
TEXT_BASE=`grep CONFIG_SYS_TEXT_BASE $B/u-boot.cfg | awk '{print $3}'`
$B/tools/mkimage -A arm -C none -T kernel -O linux \
	-d $O/u-boot.bin -a $TEXT_BASE -e $TEXT_BASE $O/u-boot.uimg

(qemu options for uimage and tftp booting)
	-net nic -net user,tftp=`pwd`
	-kernel u-boot.uimg

(u-boot commands)
dhcp 0x61000000 vexpress-v2p-ca9.dtb
dhcp 0x62100000 zImage
setenv bootargs loglevel=7 console=ttyAMA0,115200 ip=dhcp root=/dev/mmcblk0p1 rootwait
bootz 0x62100000 - 0x61000000