ANDROID: Don't add image to all target with KBUILD_MIXED_TREE

When KBUILD_MIXED_TREE is set, vmlinux and related images come from an
out-of-tree location and vmlinux shouldn't be compiled. arch/arm64 and
arch/x86 add the default Images to all target, so remove that default
behavior when KBUILD_MIXED_TREE is set.

This when an out-of-tree module, e.g. virtual-device, runs "make all"
instead of "make modules".

Bug: 178469391
Change-Id: I8c43d5d66a8bf6ed27f91df7e173399d4f4a23c0
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>