base-files: upgrade: Add support for block devices like eMMC

Add functions determining type of target flashed object and pass it down to iopsys_write_fit_image_component() functions.

Depending on:

• current root device type (emmc/nand),
• target area type (boot/rootfs/bootloader),
• target reference type (label/device)

returns:

• target device type: (mtd/ubi/blockpart/blockdev)

Change __write_fit_image_component_to_emmc() to __write_fit_image_component_to_blockpart() as this function is not eMMC specific and blockpart fits better for functions determining write type.

If no loader_device parameter for iopsys_do_upgrade() is supplied then label device type with "u-boot" label is assumed. This may be a bit generalized using get_default_loader_device I think.

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Fixed some noticed typos