ENGR00243038: make get_mmc_env_devno to be a generic function

Currently, each board has one same function called get_mmc_env_devno,
this will make the code a little bit duplication. We can make the
get_mmc_env_devno to be a generic function, thus we can remove all the
scattered function definition in each board file.

And the patch also remove the boot check. Firstly, this check is needless,
secondly, this will break the second boot support,for example:
first boot from SPI, then switch to SD/MMC boot.

Signed-off-by: Jason Liu <r64343@freescale.com>