arch/x86/purgatory/Makefile: try to use automatic variable in kexec purgatory makefile

Make the Makefile of kexec purgatory be consistent with others in linux
src tree, and make it look generic and simple.
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/purgatory/Makefile

@@ -18,8 +18,9 @@ $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
 
 targets += kexec-purgatory.c
 
+CMD_BIN2C = $(objtree)/scripts/basic/bin2c
 quiet_cmd_bin2c = BIN2C   $@
-      cmd_bin2c = cat $(obj)/purgatory.ro | $(objtree)/scripts/basic/bin2c kexec_purgatory > $(obj)/kexec-purgatory.c
+      cmd_bin2c = $(CMD_BIN2C) kexec_purgatory < $< > $@
 
 $(obj)/kexec-purgatory.c: $(obj)/purgatory.ro FORCE
 	$(call if_changed,bin2c)