[ARM] pxa: PXA_SHARPSL_DETECT_MACH_ID to include head-sharpsl.S

With kexec-based kernel boot loader on Zaurus, the machine ID is
actually correctly passed, and head-sharpsl.S is not necessary.
Introduce PXA_SHARPSL_DETECT_MACH_ID, and include head-sharpsl.S
only when that's explicitly enabled.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 53faa90..2ea1bfa 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -36,7 +36,7 @@
 OBJS		+= head-xscale.o
 endif
 
-ifeq ($(CONFIG_PXA_SHARPSL),y)
+ifeq ($(CONFIG_PXA_SHARPSL_DETECT_MACH_ID),y)
 OBJS		+= head-sharpsl.o
 endif