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

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 <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>