[brcm63xx] flashmap and image generation: reduced union bcm_tag to a single struct

[brcm63xx] flashmap and image generation: reduced union bcm_tag to a single struct
combining the elements so that it is no longer necessary to create an openwrt-only
tagid and tagcrc, and elimate the tagid detection and switch statements which
made dealing with imagetags overly complicated, especially since the logic would
need analogs in all code that touched the imagetag. Patch from cshore.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@20652 3c298f89-4303-0410-b956-a3cf2f4a3e73