This patch intend to fix the random unpack failure of linux-libc-headers-yocto
and linux-yocto.
The root cause of the unpack failure is that: these two recpies has the same URL, thus
has the same dest file during the fetch and unpack phase:
do_fetch : create tar ball ${DL_DIR}/git_git.pokylinux.org.linux-yocto-2.6.37.tar.gz
do_unpack : extract tar ball ${DL_DIR}/git_git.pokylinux.org.linux-yocto-2.6.37.tar.gz
fetch phase is protected by lockfile, so it works fine. but unpack phase is not lock protected,
thus there is race condition like: when linux-yocto do_unpack is extracting the tar ball,
linux-libc-headers-yocto do_fetch starts to create tar ball thus overwrite linux-yocto's
tar ball and cause linux-yocto do_unpack failure
To fix this issue, do_unpack also need to be protected by lock
