netboot: fix dependency loop for qemu build

netboot: fix dependency loop for qemu build

qemu images for 64-bit x86 doesn't generate a u-boot image
however netboot does by default. Fix this issue by checking for
vmdk in image types.

Change-Id: I061d61c87c6be9c16a04926d56b0825a3eedde04
Bug-AGL: SPEC-513
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10125
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>