tags and cscope support really belongs in a shell script

tags and cscope support really belongs in a shell script
as they do not benefit from the make functionality.

Moving the support to a shell script has several benefits:
- The readability of the code has increased a lot
- More people is able to extend the tags support
- We see less changes to the top-level Makefile

The shell script version includes improvements from:
Alexey Dobriyan <adobriyan@gmail.com> (jump to kconfig symbols)
Alexey Dobriyan <adobriyan@gmail.com> (drop ./ in paths)
Ian Campbell <ijc@hellion.org.uk> (simplified find algorithms)

This version has a few caveats:
=> It does not support ALLSOURCE_ARCHS
   - it is easy to add if it is really used
=> It assumes all archs have moved to arch/$ARCH/include
   - until that happens we have a few additional hits in the archs

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Tested-by: Ian Campbell <ijc@hellion.org.uk>