package/bash: add option to keep or remove loadable examples

package/bash: add option to keep or remove loadable examples

bash has a concept of "loadables", which are "plugins" that can be
loaded at runtime by bash to add new builtin. For example:

    # type whoami
    whoami is hashed (/usr/bin/whoami)
    # whoami
    root

    # enable -f /usr/lib/bash/whoami whoami
    # type whoami
    whoami is a shell builtin
    # whoami
    root

    # enable -d whoami
    # type whoami
    whoami is hashed (/usr/bin/whoami)
    # whoami
    root

bash comes with a set of example loadables, installed in
/usr/lib/bash/. They take 312 KB on ARM32, and are by default not
used, and provide builtins that are for the most part already
available as external commands in Busybox/coreutils:

    Makefile.inc  finfo         mkfifo        realpath      sync
    accept        head          mktemp        rm            tee
    basename      id            mypid         rmdir         truefalse
    csv           ln            pathchk       seq           tty
    cut           loadables.h   print         setpgid       uname
    dirname       logname       printenv      sleep         unlink
    fdflags       mkdir         push          strftime      whoami

So instead of having them unconditionally installed, add an option to
enable/disable their installation (their build apparently cannot be
disabled via a configure option).

Normally, we try to keep backward compatibility by preserving the
existing behavior. In this case, this would have meant making this
option "default y". But this also breaks our principle of "being
minimal by default", and in this case, it feels preferable to be
"minimal by default" than preserving existing behavior.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>