fetch2: fix unpack of .xz files

If a file ends with .xz, it currently gets overwritten during unpack:
The decompress command for .xz files is:
  'xz -dc %s > %s' % (file, efile)
and as efile == file, we end up overwriting file (the source).

Fix this by adding .xz to the list of suffixes that that need to
be removed from a file name for an extract command, leaving the
bare file name. Now, for a given file foo.xz,
file == foo.xz and efile == foo, similar to how .gz .bz2 and .Z
files are treated.

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake-worker: import needed signal module

bitbake-worker makes use of the signal module
but it doesn't import it. This patch fixes the issue.

[YOCTO #4750]

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>