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.

(From OE-Core rev: 21d7ab8ce10f6d6a56875244c09dcfebae457b22)

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit fbcf2c1c255b0c61a795c032cf7b67f5db41baa8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/image.bbclass

@@ -654,7 +654,7 @@ reproducible_final_image_task () {
     if [ "${BUILD_REPRODUCIBLE_BINARIES}" = "1" ]; then
         if [ "$REPRODUCIBLE_TIMESTAMP_ROOTFS" = "" ]; then
             REPRODUCIBLE_TIMESTAMP_ROOTFS=`git -C "${COREBASE}" log -1 --pretty=%ct 2>/dev/null` || true
-            if [ "${REPRODUCIBLE_TIMESTAMP_ROOTFS}" = "" ]; then
+            if [ "$REPRODUCIBLE_TIMESTAMP_ROOTFS" = "" ]; then
                 REPRODUCIBLE_TIMESTAMP_ROOTFS=`stat -c%Y ${@bb.utils.which(d.getVar("BBPATH"), "conf/bitbake.conf")}`
             fi
         fi