s390/boot: enable .bss section for compressed kernel

- Support static uninitialized variables in compressed kernel.
- Remove chkbss script
- Get rid of workarounds for not having .bss section

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>