Boot default fit image config

[Ham22]

Currently we boot to a hardcoded fit config which means we have to set it for different dists e.g. buildroot and openwrt.

We should instead expect those buildsystems to label a default config and boot from that.

This depends on CreatorDev/openwrt#218

Acceptance criteria:

• by default will boot to the 'default' config defined in the fit image
• still allows a user to override which config to boot from by setting fitconf variable
• successfully boots the default set for buildroot and openwrt images