Fix compilation warnings in cmd_fastboot.c

There were multiple warnings compiling with gcc 4.9:
  -- arguments mismatching prototypes
  -- missing prototypes
  -- printf format strings not matching arguments
  -- unused variables

This patch cleans them all up.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>
use bootm not bootz for fastboto boot

do_bootz only works to boot zImage-style images; using bootm will
adapt to the image type, and allow ELF, Raw and zImage images.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>
Use bootz for zIMage and raw images.

You can do, say,
   fastboot -b 43DF8000 boot u-boot.bin
to boot a raw image.  The base address (-b argument) must be 0x8000 below the
starting address if you don't have position independent code.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>
Allow use of fastboot on non-Android systems

This change allows fastboot to be used to flash boot partitions and
boot images even on a non-Android layout.  Rather than exit when a
non-android partitioning scheme is found when the user types
`fastboot' to the u-boot prompt, rely on checking that a partition
exists at flash or erase time.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>