Remove CONFIG_SYS_MMC_IMG_LOAD_PART from CONFIG namespace

This option is used as part of configuring the default environment for a
number of platforms.  However, it is always set to 1 and the only time
it is part of Kconfig, it is used in a hard-coded manner.  Hard-code the
value in the environment instead.

Signed-off-by: Tom Rini <trini@konsulko.com>