Facilitate '-j' when building kernel

thanks for the input of @tianon and @steeve

Signed-off-by: Tzu-Jung Lee <roylee17@gmail.com>
Merge pull request #45 from roylee17/build-kernel-in-parallel

Facilitate '-j' when building kernel
Add the real xz package. Closes #126
Merge remote-tracking branch 'upstream/master'

Conflicts:
	rootfs/Dockerfile