kernel.mk: fix kernel targets for arm64

Use the Image.gz kernel target for arm64 to be able append DTB to kernel
bin at the end.

BUG: 27674989

Change-Id: I795db9dd235d14d679904e9e3d8074f19ee757f1
Signen-nff-by: Dzmitry Yatsushkevich <dmitryya@nestlabs.com>
1 file changed