image.bbclass: fix REPRODUCIBLE_TIMESTAMP_ROOTFS reference
Commit
97b439469a45a089431ca9c31893288c855045f4 added a fallback
mechanism for getting the rootfs timestamp. However, it uses curly
braces around the variable name, which causes bitbake resolve the
variable reference, rather than the shell, so the git timestamp
never gets used. Fix the reference to restore the intent of
making it a fallback for when there is no git timestamp to
retrieve.
Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
