s390/boot: Rename decompressor_printk() to boot_printk()

Rename decompressor_printk() to boot_printk() just to have a shorter
function name, which also makes the code more readable.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>