rootfs: Make BUILDNAME a weak default in reproducible_build_simple
11e45082ad00 ("rootfs-postcommands.bbclass: improve binary
reproducibility") fixed binary reproducibility of /etc/version, but with
the move to reproducibilty in all builds, setting /etc/version to
anything other than the default fixed timestamp is tricky because
rootfs_reproducible() runs very late.
rootfs.py uses BUILDNAME if set for /etc/version, so introduce a weak
default for BUILDNAME of "REPRODUCIBLE_TIMESTAMP_ROOTFS", when enabling
reproducible builds hence allowing BUILDNAME to be overridden elsewhere.
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
