omb_utils: handle nandwrite/nanddump parameters for dreambox

When OMB_DREAMBOX is defined use nandwrite/nanddump parameters for dreambox.
We can define OMB_DREAMBOX using the following in bitbake:

${@base_contains("MACHINE_FEATURES", "dreambox", "-DOMB_DREAMBOX", "", d)}
Merge pull request #9 from athoik/dreambox

omb_utils: handle nandwrite/nanddump parameters for dreambox