m68k: coldfire: fix warnings in uboot argument processing

When building with W=1:

      CC      arch/m68k/kernel/uboot.o
    arch/m68k/kernel/uboot.c: In function ‘parse_uboot_commandline’:
    arch/m68k/kernel/uboot.c:68:36: warning: variable ‘uboot_initrd_end’ set but not used [-Wunused-but-set-variable]
      unsigned long uboot_initrd_start, uboot_initrd_end;
                                        ^~~~~~~~~~~~~~~~
    arch/m68k/kernel/uboot.c:68:16: warning: variable ‘uboot_initrd_start’ set but not used [-Wunused-but-set-variable]
      unsigned long uboot_initrd_start, uboot_initrd_end;
                    ^~~~~~~~~~~~~~~~~~
    arch/m68k/kernel/uboot.c:66:16: warning: variable ‘uboot_kbd’ set but not used [-Wunused-but-set-variable]
      unsigned long uboot_kbd;
                    ^~~~~~~~~
    arch/m68k/kernel/uboot.c: At top level:
    arch/m68k/kernel/uboot.c:90:13: warning: no previous prototype for ‘process_uboot_commandline’ [-Wmissing-prototypes]
     __init void process_uboot_commandline(char *commandp, int size)
                 ^~~~~~~~~~~~~~~~~~~~~~~~~

A couple of issues here. Firstly we already have a bootinfo.h that has a
prototype for process_uboot_commandline(), we should include that.
Secondly uboot_kbd is not used at all and can be removed. Thirdly the
conditional code based on CONFIG_BLK_DEV_INITRD means that sometimes
uboot_initrd_start and uboot_initrd_end are not needed. Make their
declaration and asignment conditional on CONFIG_BLK_DEV_INITRD same as
the code that uses them.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>