Prevent loader.conf load failure due to unknown console entries

When processing loader.conf if console contained an entry for an unsupported
console then cons_set would return an error refusing to set any console.

This has two side effects:

1. Forth would throw a syntax error and stop processing loader.conf at that point.
2. The value of console is ignored.

#1 Means other important loader.conf entries may not be processed, which is

clearly undesirable.

#2 Means the users preference for console aren't applied even if they did

contain valid options. Now we have support for multi boot paths from a
single image e.g. bios and efi mode the console preference needs to deal
with the need to set preference for more than one source.

Fix this by:

• Returning CMD_OK where possible from cons_set.
• Allowing set with at least one valid console to proceed.

Reviewed by: allanjude
MFC after: 1 week
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D5018