s390/ipl: Use diagnose 8 command separation

Currently the vmcmd shutdown action is parsed by the kernel and
if multiple cp commands have been specified, they are issued
separately with the cpcmd() function.

The underlying diagnose 8 instruction already allows to specify
multiple commands that are separated by 0x15. The ASCEBC() function
used by cpcmd() translates '\n' to 0x15. The '\n' character is
currently used as vmcmd command separator and therefore the vmcmd
string can be passed directly to the cpcmd() function.

Using the diagnose 8 command separation has the advantage that also
after disruptive commands that stop Linux, for example "def store",
additional commands can be executed.

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