mkimage: Make table_entry code global

- make get_table_entry_id() global
- make get_table_entry_name() global
- move struct table_entry to image.h

Currently this code is used by image.c only.

This patch makes this API global so it can be used by other parts of
code, too.

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Ron Lee <ron.debian.org>

Edit comments and commit message.

Signed-off-by: Wolfgang Denk <wd@denx.de>