Currently Kconfig dependencies are used to allow non-interactive builds.
This leads to problems in Kconfig getting the dependencies right.

This patch adds a barebox_main function pointer which is called at the
end of the startup process. This defaults to run_shell when a shell is
enabled.

With this the HAVE_NOSHELL Kconfig variable can be removed. Non interactive
builds can now be enabled for every board allowing to compile a binary
without further Kconfig dependencies. This also allows for more flexibility,
for example boards may decide to try non-interactive startup first and
call run_shell if that fails.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>