Makefile: Remove KERNEL_IMG #1827

Job	Run time
packages (arm64-secure, arm64)	4m 0s
packages (ubuntu-20.04, amd64)	12m 21s
packages (ubuntu-latest, riscv64)	4m 1s
eve (arm64, kvm)	38m 58s
manifest	4m 32s
eve (arm64, xen)	37m 20s
verification (arm64, kvm)	11m 59s
eve (amd64, kvm)	34m 34s
verification (arm64, xen)	11m 37s
eve (amd64, xen)	33m 31s
verification (amd64, kvm)	14m 17s
eve (riscv64, mini)	5m 36s
verification (amd64, xen)	13m 37s
trigger_assets	0s
	3h 46m 23s

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