m68k, kexec: fix the incorrect ifdeffery and build dependency of CONFIG_KEXEC

The select of KEXEC for CRASH_DUMP in kernel/Kconfig.kexec will be
dropped, then compiling errors will be triggered if below config items are
set:

===
CONFIG_CRASH_CORE=y
CONFIG_KEXEC_CORE=y
CONFIG_CRASH_DUMP=y
===

Here, change the dependency of buinding machine_kexec.o relocate_kernel.o
and the ifdeffery in asm/kexe.h to CONFIG_KEXEC_CORE.

Link: https://lkml.kernel.org/r/20231208073036.7884-3-bhe@redhat.comSigned-off-by: Baoquan He <bhe@redhat.com>
Cc: Eric DeVolder <eric_devolder@yahoo.com>
Cc: Ignat Korchagin <ignat@cloudflare.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/m68k/include/asm/kexec.h

@@ -2,7 +2,7 @@
 #ifndef _ASM_M68K_KEXEC_H
 #define _ASM_M68K_KEXEC_H
 
-#ifdef CONFIG_KEXEC
+#ifdef CONFIG_KEXEC_CORE
 
 /* Maximum physical address we can use pages from */
 #define KEXEC_SOURCE_MEMORY_LIMIT (-1UL)
@@ -25,6 +25,6 @@ static inline void crash_setup_regs(struct pt_regs *newregs,
 
 #endif /* __ASSEMBLY__ */
 
-#endif /* CONFIG_KEXEC */
+#endif /* CONFIG_KEXEC_CORE */
 
 #endif /* _ASM_M68K_KEXEC_H */

arch/m68k/kernel/Makefile

@@ -25,7 +25,7 @@ obj-$(CONFIG_PCI) += pcibios.o
 
 obj-$(CONFIG_M68K_NONCOHERENT_DMA) += dma.o
 
-obj-$(CONFIG_KEXEC)		+= machine_kexec.o relocate_kernel.o
+obj-$(CONFIG_KEXEC_CORE)	+= machine_kexec.o relocate_kernel.o
 obj-$(CONFIG_BOOTINFO_PROC)	+= bootinfo_proc.o
 obj-$(CONFIG_UBOOT)		+= uboot.o