frv: clean up arch/frv/Makefile

- removed a lot of unused variable assignmnets
- removed unused bootstrap target
- replaced ARCHMODFLAGS with proper KBUILD_{A,C}FLAGS_MODULE assignmnets

The resuting Makefile has not been tested due to lack of toolchain,
but these were all trivial changes.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>