s390/boot: cleanup decompressor header files

Move declarations to appropriate header files. Instead of cryptic
casting directly assign struct vmlinux_info type to _vmlinux_info
linker script variable - wich it actually is.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

s390/boot: get rid of startup archive

The final kernel image is created by linking decompressor object files with
a startup archive. The startup archive file however does not contain only
optional code and data which can be discarded if not referenced. It also
contains mandatory object data like head.o which must never be discarded,
even if not referenced.

Move the decompresser code and linker script to the boot directory and get
rid of the startup archive so everything is kept during link time.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>