Use correct value pointing to previously selected FDT DTB.

Reviewed by:	brooks
Sponsored by:	DARPA, AFRL
MFC after:	1 week
Fix multi-repository support by properly respecting 'enabled' flag.

This will read the REPOS_DIR env/config setting (default is /etc/pkg
and /usr/local/etc/pkg/repos) and use the last enabled repository.

This can be changed in the environment using a comma-separated list,
or in /usr/local/etc/pkg.conf with JSON array syntax of:
    REPOS_DIR: ["/etc/pkg", "/usr/local/etc/pkg/repos"]

Approved by:	bapt
MFC after:	1 week
Add an FDT DTS and MDROOT kernel configuration for BERI on NetFPGA.

At this point we only support one CPU, the PIC, and a UART console.

Reviewed by:	brooks
Sponsored by:	DARPA, AFRL
MFC after:	5 days