system: add option to choose what /bin/sh points to

Not all our shells do install a pointer to /bin/sh. Besides, between
those that do and multiple ones are enabled, the last one to install
wins the the symlink.

Add a new config choice in the system sub-menu that allows the user to
explicitly select the shell to provide /bin/sh. If busybox is not
enabled, default to using dash, a POSIX shell.

Remove the symlink creation from bash.mk at the same time.

Note: for every shell, we select them, except busybox, on which we
depend, on the assumption that we do not want to force busybox in case
the user decided not to enable it.

[Peter: remove redundant !BR2_PACKAGE_BUSYBOX dependency]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>