Add debian minimal requirement for arm64

Debian system building fails, fixed by adding arm64 package.

DIB_RELEASE=jessie disk-image-create debian ironic-agent \
    grub2 devuser -a arm64 -o deploy-jessie
dib-run-parts Running /tmp/dib_build.v5FEtaKx/hooks/cleanup.d/99-extract-kernel-and-ramdisk
ls: cannot access /tmp/dib_build.v5FEtaKx/mnt/boot/vmlinu*: No such file or directory

Change-Id: I610d767785df49fed954f12854be5ae78ff9baa6
Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>

diskimage_builder/elements/debian-minimal/package-installs.yaml

@@ -2,4 +2,6 @@ linux-image-amd64:
   arch: amd64
 linux-image-686:
   arch: i386
+linux-image-arm64:
+  arch: arm64
 netbase: