mx*sabresd: Reference CONFIG_SYS_AUXCORE_BOOTDATA value directly

As this is used in the environment, reference it directly rather than as
a CONFIG value.

Cc: Fabio Estevam <festevam@gmail.com>
Cc: Adrian Alonso <adrian.alonso@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>