rootfs-postcommands.bbclass: improve binary reproducibility

rootfs-postcommands.bbclass: improve binary reproducibility

Conditionally support binary reproducibility of rootfs images.
If REPRODUCIBLE_TIMESTAMP_ROOTFS is specified then:

1. set /etc/timestamp to a reproducible value
2. set /etc/version to a reproducible value
3. set /etc/gconf: set mtime in all %gconf.xml to reproducible values

The reproducible value is taken from the variable REPRODUCIBLE_TIMESTAMP_ROOTFS.

[YOCTO #11176]
[YOCTO #12422]

(From OE-Core rev: 11e45082ad00b9c172e59bf6b2a76dd613773f5a)

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>