bitbake: build: filter out expanded empty strings for lockfiles flag

bitbake: build: filter out expanded empty strings for lockfiles flag

If we have something similar to:

    LOCKFILES = "${TMPDIR}/my.lock"
    LOCKFILES_qemuall = ""

    do_task[lockfiles] += "${LOCKFILES}"

when expanded, lockfiles will be empty for qemu, resulting in

     File "/home/stefans/yocto/poky/bitbake/lib/bb/utils.py", line 630, in mkdirhier
	raise e
    OSError: [Errno 2] No such file or directory: ''

This should filter out the empty expansions.

(Bitbake rev: 7813e1bfd08cd48871f8c03cae2810265590105d)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>