makefile: change the dependency of "kernel" target
makefile: change the dependency of "kernel" target

Because the "bootimg" target is not available,
so change the dependency of "kernel" target to
"virtual/kernel"

Tracked-On: EDISON-2587
Signed-off-by: WangBo <0abe653db35042b08c02ad26c438c9cc3caeac45@intel.com>
Signed-off-by: Zhenming Zhao <7b6ce0b13ca94a2cf6687af033670cf3f14a2959@intel.com>

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