This allows to build a custom Talos image which comes with some system
extension bundled in. Sometimes we might need to have an extension in
the initial image, e.g. `vmtoolsd` for VMWare Talos image.
Syntax:
```
make image-aws \
IMAGER_SYSTEM_EXTENSIONS="ghcr.io/siderolabs/amd-ucode:..."
```
System extensions are not supported for now for ISO images, as they
don't go through the common installer flow (#5725).
Also it might be nice to add a simple way to generate just
`initramfs.xz` with system extensions bundled in (e.g. for PXE booting).
(#5726)
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
