[new uImage] Factor out common routines for getting os/arch/type/comp names

Move numeric-id to name translation for image os/arch/type/comp header
fields to a helper routines: image_get_os_name(), image_get_arch_name(),
image_get_type_name(), image_get_comp_name().

Signed-off-by: Marian Balakowicz <m8@semihalf.com>