MFC r323206: Enable dtrace support for mips64 and the ERL kernel config

Turn on the required options in the ERL config file, and ensure
that the fbt module is listed as a dependency for mips in
the modules/dtrace/dtraceall/dtraceall.c file.

MFC r307152: Add COMPAT_FREEBSD10 to the MIPS ERL kernel config

As of r302092, pipe is a wrapper around pipe2 and the pipe syscall is no
longer used. It is included only with the COMPAT_FREEBSD10 kernel option.
Add the compat option to support upgrades from systems with an earlier
userland.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add Ubiquiti EdgeRouter Lite (ERL) kernel config file

The ERL is a fairly cheap (~$100 USD) and readily available dual core
MIPS64 device so it makes a useful MIPS reference platform.

This is based in part on the kernel config generated by the mkerlimage
script from http://rtfm.net/FreeBSD/ERL/.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3884