Add new functions require_binaries() and require_any_binary() to be used
in the check() section of module-setup.sh.
These functions print a warning line telling the user, which binary is
missing for the specific dracut module.
This unifies the way of checking for binaries and makes the life of an
initramfs creator easier, if he wants to find out why a specific dracut
module is not included in the initramfs.
"inst busybox" no longer seems to work when busybox is at
/sbin/busybox. Reproduced on Fedora 18 (dracut-023-39.git20120910).
Use type -P to find the full path to busybox to solve this problem.
Currently in dracut busybox module, we hard-code the list of binaries
available in busybox. Actually we can get this list by running busybox
without any arguments.
Also, if find_binary() finds nothing, we would symlink $initdir/
which is not we expect.
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>