malta: convert to Device macro image building

malta: convert to Device macro image building

Convert the malta target from the legacy Image/BuildKernel and
Image/Build pattern to the modern Device macro system. This is the
last target still using the legacy pattern.

The Device macro system automatically generates per-image JSON
metadata files which get aggregated into profiles.json, enabling
firmware selector and other tooling support for all malta subtargets
(be, le, be64, le64).

The kernel ELF is produced via KERNEL_NAME := vmlinux.elf (matching
octeon), uImage artifacts are built using the standard Build/lzma,
Build/gzip and Build/uImage commands with the existing load address
0x80100000, and rootfs images use append-rootfs with optional gzip
compression.

The device is named 'generic' following the convention used by other
virtual/emulated targets (x86, armsr, octeon).

Signed-off-by: Paul Spooren <mail@aparcar.org>