Until now, ‘channel-build-system’ would assume a single channel, the
‘guix’ channel. This change lets users specify additional channels
using the #:channels parameter.
* guix/build-system/channel.scm (build-channels): Add #:channels and
honor it.
(channel-build-system): In ‘lower’, add #:channels and honor it.
* doc/guix.texi (Build Systems): Document it.
Change-Id: I36c1d19cbeee02a4d1144de089b78df0390774a0
d6a3818761