allmodconfig and allyesconfig are currently failing because they select
wanxl firmware rebuild, and that requires an m68k assembler toolchain.

Add a new generic option to the "Generic Driver Options" menu.  The option
is defaul equals y and prevents us from building firmware unless really
needed.

Firmware is usually provided in separate filer '_shipped', and there is no
need to rebuild them unless strictly required.  First user is WanXL - which
otherwise required a m68k tool-set to compile.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>