':' is the null command, sometimes used in shell scripts (especially
after 'then'), but it is not yet supported by barebox.  It is supposed
to do nothing, successfully, just like the 'true' command, so add it as
an alias.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>