Need omicron-package preset for a4x2

[plotnick]

The a4x2 testbed will need a new target preset to build its packages (see oxidecomputer/testbed#89). It must include both -m non-gimlet and -r multi-sled, and so does not match any of the presets defined in #7288.

[sunshowers]

(Note you can pass in overrides manually, but the preset is definitely better)