selftest: DiskMonTest: use POSIX output for df

Without "-P" df  splits the output on multiple lines and breaks the test

Signed-off-by: Alexandru Palalau <alexandrux.palalau@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
sstate: Silence warnings when switching machines

We recently added multiple provider warnings. These could get issued
from -cross recipes due to the way these currently overlap in the
native sysroot. Filter out these warnings for now, until such times
as we improve the cross recipes so they don't have an overlapping
namespace.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>