s390/decompressor: trim the kernel image up to 1M

Move head64.S main kernel entry point "startup_continue" to 0x100000 and
trim everything which is below 1M during build. So, that the decompressor
would unpack the main kernel image, move it to 0x100000 and jump to
startup_continue.

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>