Bug#794538: devscripts: mk-build-depends generates wrong dependency for cross builds

[Dima Kogan]

Package: devscripts
Version: 2.15.1
Severity: normal

mk-build-deps has a -a option to target foreign arches. This however
generates a dependency build-essential:$arch. This is WRONG for
cross-building. In the current world we would want
crossbuild-essential-$arch:native, but this is a bit controversial
currently, and could change in the future. How about simply omitting the
build-essential dependency if -aforeign is given?