xtensa: use uppercase for configurations and modified overlay structure

Except for architecture and processor names, buildroot uses capitalized
configuration names, so change the macro names for xtensa to follow that
standard.
Change the overlay file to have a subdirectory for each component
(gdb, binutils, gcc, etc.) to make it more future-prove.

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>