Fix printf() format problems with configurable prompts

U-Boot allows for configurable prompt strings using the
CONFIG_AUTOBOOT_PROMPT resp. CONFIG_MENUPROMPT definitions. So far,
the assumption was that any such user defined problts would contain
exactly one "%d" format specifier. But some boards did not.

To allow for flexible boot prompts without adding too complex code we
now allow to specify the whole list of printf() arguments in the user
definition. This is powerful, but requires a responsible user who
really understands what he is doing, as he needs to know for exanple
which variables are available in the respective context.

Signed-off-by: Wolfgang Denk <wd@denx.de>