powerpc: Copy bootable images in the default install script

This patch makes the default install script (arch/powerpc/boot/install.sh)
copy the bootable image files into the install directory.  Before this
patch only the vmlinux image file was copied.

This patch makes the default 'make install' command useful for embedded
development when $(INSTALL_PATH) is set in the environment.

As a side effect, this patch changes the calling convention of the
install.sh script.  Instead of a single 5th parameter, the script is now
passed a list of all the target images stored in the $(image-y) Makefile
variable.  This should be backwards compatible with existing install scripts
since it just adds additional arguments and does not change existing ones.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>