Implement partition type selection for upload and download

[Juul]

There are two layers of partitions handled in u-boot. The partitions are specified at the top of /include/asm-arm/arch-mx50/mx50_yoshi_board.h in the boards[] array:

static const struct board_type boards[] = ...

Each entry looks like e.g:

.name = "userdata",
.address = CONFIG_MMC_USERDATA_ADDR,
.size = CONFIG_MMC_USERDATA_SIZE,  /* 5 KiB */
.partition = 1,

Not only does the partition have an address, it also has a partition number. This number refers to one of three meta-partitions (0, 1 or 2) that are selected using MMC commands.

The upload and download commands need a feature to specify which of the three to select.