s390/zfcpdump: Update documentation in zfcpdump.txt

Do the following changes:

 - Document new /proc/vmcore interface
 - Document partition dump external initramfs with s390-tools-1.24.0
 - Remove initramfs config file because initramfs is now built automatically
   in s390-tools
 - Replace description of kernel config options with "make zfcpdump_defconfig"
 - Some editorial changes

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>