s390/boot: add zstd support

Enable ztsd support in s390/boot, to enable booting with zstd
compressed kernel when configured with CONFIG_KERNEL_ZSTD=y.

BOOT_HEAP_SIZE is defined to 0x30000 in this case. Actual decompressor
memory usage with allyesconfig is currently 0x26150.

BugLink: https://bugs.launchpad.net/bugs/1931725
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
cc: Heiko Carstens <hca@linux.ibm.com>
cc: Vasily Gorbik <gor@linux.ibm.com>
cc: Christian Borntraeger <borntraeger@de.ibm.com>
cc: linux-s390@vger.kernel.org
Link: https://lore.kernel.org/r/20210615114150.325080-1-dimitri.ledkov@canonical.com
[gor: added BOOT_HEAP_SIZE for zstd]
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>