[AVR32] rename vmlinux.lds

Rename vmlinux.lds to a .S file to match other architectures.
Simplify Makefile to match the rename and deleted the unused
USE_STANDARD_AS_RULE

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>