swupdate-common: split do_swuimage in smaller function
swupdate-common: split do_swuimage in smaller function

Split do_swuimage in smaller functions, easier to understand and to
maintain.

Signed-off-by: Stefano Babic <2c2905e0c2877c7e4ce40c7425e972302953fda5@denx.de>

sbabic/meta-swupdate,sbabic/meta-swupdate