s390/purgatory: Remove duplicate variable definitions

Currently there are some variables in the purgatory (e.g. kernel_entry)
which are defined twice, once in assembler- and once in c-code. The reason
for this is that these variables are set during purgatory load, where
sanity checks on the corresponding Elf_Sym's are made, while they are used
in assembler-code. Thus adding a second definition in c-code is a handy
workaround to guarantee correct Elf_Sym's are created.

When the purgatory is compiled with -fcommon (default for gcc on s390) this
is no problem because both symbols are merged by the linker. However this
is not required by ISO C and when the purgatory is built with -fno-common
the linker fails with errors like

arch/s390/purgatory/purgatory.o:(.bss+0x18): multiple definition of `kernel_entry'
arch/s390/purgatory/head.o:/.../arch/s390/purgatory/head.S:230: first defined here

Thus remove the duplicate definitions and add the required size and type
information to the assembler definition. Also add -fno-common to the
command line options to prevent similar hacks in the future.

Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

s390/kexec_file: add declaration of purgatory related globals

Fix the following sparse complaints:

arch/s390/purgatory/purgatory.c:18:5: warning: symbol 'kernel_entry' was not declared. Should it be static?
arch/s390/purgatory/purgatory.c:19:5: warning: symbol 'kernel_type' was not declared. Should it be static?
arch/s390/purgatory/purgatory.c:21:5: warning: symbol 'crash_start' was not declared. Should it be static?
arch/s390/purgatory/purgatory.c:22:5: warning: symbol 'crash_size' was not declared. Should it be static?

Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

s390/kexec_file: Add purgatory

The common code expects the architecture to have a purgatory that runs
between the two kernels. Add it now. For simplicity first skip crash
support.

Signed-off-by: Philipp Rudo <prudo@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>