core-image-minimal-initramfs-alix3d3: add initramfs image

Add a specific initramfs image for ALIX3D3 board. The initramfs has
initramfs-framework-base and udev-module.

ALIX3D3 image is formatted with two partitions. The first has boot, and
the second partition has rootfs. In this case,
core-image-minimal-initramfs will not work as it depends on live init
scripts, which in turn looks for rootfs.img file and mounts it into
/dev/ram0
classes, alix3d3.conf: Add IMAGE_CMD_alix-hddimg command

Add a bbclass for creating hdd images with 2 partitions. The first
partition has syslinux, initrd, and kernel, the second partition has
rootfs. The produced image can be directly dd'ed into CF card and run.

Since /dev/sda2 is mounted as root, the changes to the system is
permanent. It is useful for daemon configuration (such as aprx).