mx6memcal: Finish migration to defconfig options

The config header for this platform uses '#undef' in a number of cases.
All of the MMC related ones were already handled correctly in the
defconfig file.  In the case of CONFIG_CMD_FUSE, the command was being
built and enabled via defconfig.  Disable it in the defconfig, cleanup
the header.

Reviewed-by: Eric Nelson <eric@nelint.com>
Signed-off-by: Tom Rini <trini@konsulko.com>