arc770: Introduce images for SD-cards

Historically on ARC we started from initramfs-based images because:
 a) It was much easier to debug especially when toolchain and other
    components were changing quite dynamically
 b) It was our usual approach for embedded Linux

But now with ARC port of Lede/OpenWRT getting more stable and mature
we're ready for more real-life scenarios with FS permanently stored
on SD-card. This essentially benefits from ability to setup devices
that survive reboots with all settings and extra packages kept in place.

Still we keep an ability to build images with initramfs.
This allows us to use storage-less simulators for testing still.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: John Crispin <john@phrozen.org>
Cc: Jo-Philipp Wich <jo@mein.io>