bitbake.conf: use ??= for IMAGE_ROOTFS_SIZE

bitbake.conf: use ??= for IMAGE_ROOTFS_SIZE

Previously, when building core-image-minimal, the rootfs size would
default to 64M because we use '?=' in bitbake.conf and also '?=' in
core-image-minimal.bb.

The thing is, we'd like to have a default value for all images set
in bitbake.conf but still allow each image recipe to set its own default
value which could be overridden by users in local.conf.

(From OE-Core rev: 18f499df6bcbf79d7bd0a99c4c8693268683485f)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>