[PATCH] show Acorn-specific block devices menu only when required

Don't show a menu that can't be entered due to lack of contents on arm (the
options are only available on arm26).

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Ian Molton <spyro@f2s.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>