main: Separate out the two abortboot() functions

There are two implementations of abortboot(). Turn these into two separate
functions, and create a single abortboot() which calls either one or the
other.

Also it seems that nothing uses abortboot() outside main, so make it static.

At this point there is no further use of CONFIG_MENU in main.c.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>