controlcenterd_36BIT_SDCARD: Fix unused variable warning

On the controlcenterd_36BIT_SDCARD config we get a warning about
prg_stage1_prepare being unused.  Move the declaration closer to usage
and hide under the existing #if tests.

Cc: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Tom Rini <trini@konsulko.com>