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.

(From OE-Core rev: bbf28ea9100a4f86b052c5cd53c77f9e1c03fb09)

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/reproducible_build_simple.bbclass

@@ -7,5 +7,6 @@ export PERL_HASH_SEED = "0"
 export SOURCE_DATE_EPOCH ??= "1520598896"
 
 REPRODUCIBLE_TIMESTAMP_ROOTFS ??= "1520598896"
+BUILDNAME ??= "${REPRODUCIBLE_TIMESTAMP_ROOTFS}"
 
 inherit podfix

meta/classes/rootfs-postcommands.bbclass

@@ -356,11 +356,6 @@ python rootfs_log_check_recommends() {
 # Perform any additional adjustments needed to make rootf binary reproducible
 rootfs_reproducible () {
 	if [ "${REPRODUCIBLE_TIMESTAMP_ROOTFS}" != "" ]; then
-		# Convert UTC into %4Y%2m%2d%2H%2M%2S
-		sformatted=`date -u -d @${REPRODUCIBLE_TIMESTAMP_ROOTFS} +%4Y%2m%2d%2H%2M%2S`
-		echo $sformatted > ${IMAGE_ROOTFS}/etc/version
-		bbnote "rootfs_reproducible: set /etc/version to $sformatted"
-
 		if [ -d ${IMAGE_ROOTFS}${sysconfdir}/gconf ]; then
 			find ${IMAGE_ROOTFS}${sysconfdir}/gconf -name '%gconf.xml' -print0 | xargs -0r \
 			sed -i -e 's@\bmtime="[0-9][0-9]*"@mtime="'${REPRODUCIBLE_TIMESTAMP_ROOTFS}'"@g'