arm64: Build kernel image when appended dtb is disabled

As KBUILD_TARGET rule is not defined for CONFIG_BUILD_ARM64_APPENDED_DTB
disabled case, Image is not generated. Add support to build the kernel
when the CONFIG_BUILD_ARM64_APPENDED_DTB is disabled.

Change-Id: I08ae193d2394c34fdd3d537c9224d3b69e81f401
Signed-off-by: Prakruthi Deepak Heragu <pheragu@codeaurora.org>