s390: remove decompressor's head.S

Decompressor's head.S provided "data mover" sole purpose of which has
been to safely move uncompressed kernel at 0x100000 and jump to it.

With current bzImage layout entire decompressor's code guaranteed to be
in a safe location under 0x100000, and hence could not be overwritten
during kernel move. For that reason head.S could be replaced with simple
memmove function. To do so introduce early boot code phase which is
executed from arch/s390/boot/head.S after "verify_facilities" and takes
care of optional kernel image decompression and transition to it.

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

arch/s390/boot/compressed/decompressor.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef BOOT_COMPRESSED_DECOMPRESSOR_H
+#define BOOT_COMPRESSED_DECOMPRESSOR_H
+
+#ifdef CONFIG_KERNEL_UNCOMPRESSED
+static inline void *decompress_kernel(unsigned long *uncompressed_size) {}
+#else
+void *decompress_kernel(unsigned long *uncompressed_size);
+#endif
+
+#endif /* BOOT_COMPRESSED_DECOMPRESSOR_H */