dash: enable line editting if libedit is selected

Otherwise, explicitly disable it, so that the configure script does not
misdetect a system-installed libedit for example.

Also, add a profile snippet enabling line editing, Emacs style.

[Peter: move DASH_INSTALL_PROFILE inside conditional,
	ensure /etc/profile.d exists]
Change-Id: I68c6dbbafa95e266860329cb9c7ff5519fda5bf8
Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>