cmd_mmc.c: Add 'partconf' command to mmc

Add a partconf sub-command to the mmc command to allow for setting
the boot_ack, boot_partition and partition_access fields of
PARTITION_CONFIG (formerly BOOT_CONFIG, EXT_CSD[179]).  Part of this
requires changing the check for 'part' from an strncmp to a strcmp, like
the rest of the sub-commands.

Cc: Andy Fleming <afleming@gmail.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>