configs/qemu_ppc_virtex_ml507: kernel build needs mkimage

Following ffbe46a5295ce4e0442728f2ad3e19afa7eaa8ed ("linux: simplify
LINUX_BUILD_CMDS"), the Linux kernel build for
qemu_ppc_virtex_ml507_defconfig builds an image format that needs
mkimage.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339544

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/qemu_ppc_virtex_ml507_defconfig

@@ -21,3 +21,6 @@ BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/qemu/ppc-virtex-ml507/linux.config"
 BR2_LINUX_KERNEL_VMLINUX=y
 BR2_LINUX_KERNEL_DTS_SUPPORT=y
 BR2_LINUX_KERNEL_INTREE_DTS_NAME="virtex440-ml507"
+
+# Kernel needs mkimage
+BR2_PACKAGE_HOST_UBOOT_TOOLS=y