classes/package_rpm: avoid bash being pulled in unnecessarily

The initial installation step for base-passwd, base-files and shadow
will pull in bash if any of these packages depend on /bin/sh and a bash
package has been built, which is undesirable if you are using busybox;
so force the early installation of one or the other depending on which
is going to be installed in the second step.

Note: this fix is not needed for 1.4 (dylan) and newer, since there we
use smart to construct the root filesystem without this initial step.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>