ANDROID: db845c: set BUILD_INITRAMFS=1

To create boot images for multiple kernels for CI testing, we
need the initramfs image. To generate initramfs, vmlinux and
System.map were also added to the list of artifacts.

Bug: 162034308
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I08fc4a45402dfff78da4d322ee70c2f63a262468