init/Kconfig: add an option for uncompressed kernel

Add "None" as the kernel compression mode.

This option is useful for debugging the kernel in slow simulation
environments, where decompressing and moving the kernel is awfully slow.

Uncompressed kernel implementation might allow early boot code to skip the
decompressor and jump right at uncompressed kernel image entry point.

Platforms implementing that should define HAVE_KERNEL_UNCOMPRESSED.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>