Revert "pvshim: make PV shim build selectable from configure"

Revert "pvshim: make PV shim build selectable from configure"

This reverts commit 8845155c831c59e867ee3dd31ee63e0cc6c7dcf2.

This upstream change changes stuff that breaks our very fragile mess
that builds the shim when it needs to, and doesn't when it should not.

The result is that it's missing in the end for the i386 build... :|

    dh_install: warning: Cannot find (any matches for)
    "usr/lib/debug/usr/lib/xen-*/boot/*" (tried in ., debian/tmp)

    dh_install: warning: xen-utils-4.14 missing files:
    usr/lib/debug/usr/lib/xen-*/boot/*
    dh_install: error: missing files, aborting