package/kexec: add optional support for xz, bump version

When xz was compiled before, kexec will use it as optional dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/sbin/kdump | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [liblzma.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

The version bump to 2.0.11 is needed to fix a configure script issue:

   line 4712: ac_fn_c_try_link: command not found

This issue was fixed by upstream commit
http://git.kernel.org/cgit/utils/kernel/kexec/kexec-tools.git/commit/configure.ac?id=58bbd468571b3e80585e5d68e97c8d67c8ed583b

,
which was merged after 2.0.9 was released.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: bump version instead of doing a "hack" to force the use of
the lzma library.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>