avr32: add .gitignore files

Ignore Kernel binaries, kernel/vmlinux.lds and a log file.
Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>

arch/avr32/boot/images/.gitignore

+uImage
+uImage.srec
+vmlinux.cso
+sfdwarf.log

arch/avr32/kernel/.gitignore

+vmlinux.lds