rootfs.py: Set PACKAGE_FEED_ARCHS when it is not defined
The PACKAGE_FEED_ARCHS is highly related to ALL_MULTILIB_PACKAGE_ARCHS, set it
automatically is better than manually, for example, we may forget to upgrade
PACKAGE_FEED_ARCHS when ALL_MULTILIB_PACKAGE_ARCHS is changed if set it
manually.
The workflow is:
Use PACKAGE_FEED_ARCHS if it is defined, if not, check DEPLOY_DIR_XXX/<arch>, add
<arch> to PACKAGE_FEED_ARCHS if it exists.
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>