build: Default to O2 optimization

Default to O2 and set Os in the AVR makefile.  Platforms besides AVR
are likely to produce better code with O2 so make that the default.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

Makefile

@@ -32,7 +32,7 @@ dirs-y = src
 cc-option=$(shell if test -z "`$(1) $(2) -S -o /dev/null -xc /dev/null 2>&1`" \
     ; then echo "$(2)"; else echo "$(3)"; fi ;)
 
-CFLAGS-y := -I$(OUT) -Isrc -I$(OUT)board-generic/ -Os -MD -g \
+CFLAGS-y := -I$(OUT) -Isrc -I$(OUT)board-generic/ -O2 -MD -g \
     -Wall -Wold-style-definition $(call cc-option,$(CC),-Wtype-limits,) \
     -ffunction-sections -fdata-sections
 CFLAGS-y += -flto -fwhole-program