mx6sabresd: Rename target to mx6sabresd_defconfig

As mx6sabresd only supports SPL target now, there is no need to
keep the 'spl' in the name of the target, so simply rename it
to mx6sabresd_defconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

diff --git a/configs/mx6sabresd_spl_defconfig b/configs/mx6sabresd_defconfig
similarity index 100%
rename from configs/mx6sabresd_spl_defconfig
rename to configs/mx6sabresd_defconfig