The bitbake codebase makes assumptions that only one copy is active
against a given build directory at a given time. This patch adds a
lockfile in TOPDIR to ensure that is the case.
Note that no unlock is needed, that is automatically dropped when
execution terminates.
(Bitbake rev: 6004cbf36c980c2574c1c9153df81a7c55317e17)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>