dfu: add 'SKIP' entity

Define a new 'SKIP' type for the DFU entities. The flashed data for that
entity is simply ignored without returning any error values.

This allows to have one flashing procedure and images for the different
board types or variants, where each board uses only the images relevant
to it and skips the rest. This is especially usefull for the THOR
protocol, which usually transfers more than one file in a single session.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>
[mszyprow: rephrased commit message and docs for easier reading, changed
	   subject to "dfu: add 'SKIP' entity"]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>