[SRU Xenial/Yakkety] LP#1581127 -- zfs: disable module checks for zfs when cross-compiling

[Andy Whitcroft]

When we test build before upload we cross-compile some architectures.
When cross-compiling we cannot build the inbuilt ZFS modules (due to
issues with the configurator).  This leads us to have missing modules
during the build leading to build failures.  This masks any other build
failures we may otherwise find in this testing.

Following this email is a bodge for the packaging to add the modules
normally produced in the ZFS builds to the modules.ignore list so that
their being missing in these test builds is tollerated but any other
missing module is still detected.

Proposing for SRU to Xenial (and for Yakkety).

-apw