mx6qsabre_common: Let mmc partition be board specific
commit 49ea0ff5 (49ea0ff5) introduced CONFIG_SYS_MMC_ENV_PART into mx6qsabresd.h
to store the mmc partition, but in order for it to have effect we should place
it into 'mmcpart' variable.
Also add CONFIG_SYS_MMC_ENV_PART into mx6qsabreauto.h.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>