[new uImage] Share common uImage code between mkimage and U-boot

This patch adds the following common routines:

1) Dedicated mkimage print_header() is replaced with common
image_print_contents()
image_print_contents_noindent()

2) Common os/arch/type/comp fields name <--> id translation routines
genimg_get_os_name()
genimg_get_arch_name()
genimg_get_type_name()
genimg_get_comp_name()
genimg_get_os_id()
genimg_get_arch_id()
genimg_get_type_id()
genimg_get_comp_id()

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