[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>