[CRIS] Build fixes for compressed and rescue images for v10 and v32:

- Use the normal cross gcc instead of using an elf specific cris toolchain.
  This removes the dependency of this second toolchain.

- Use the normal cross objcopy instead of overriding it to use elf-toolchain.
  This allows compiling using "CROSS_COMPILE=$CRIS_GCC/cris-axis-linux-gnu-"
  instead of just "CROSS_COMPILE=$CRIS_GCC/cris-axis-linux-gnu/bin/"

- Remove redundant rules for compiling, the implicit rules are sufficient.

- Convert the arch/cris/arch-v10/boot/compressed/head.S to format
  accepted by the cris-axis-linux-gnu-gcc (registers must be prefixed
  with '$', remove explicit underscore on exported symbols)

- Remove a number of unused (and duplicated) prototypes from
  arch/cris/arch-v10/boot/compressed/misc.c.

- Correct memcpy and memset return values (actually return them!)

Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>