avr32: move CONFIG_AVR32 definition to arch/avr32/config.mk

Like other architectures, CONFIG_AVR32 can be defined
in arch/avr32/config.mk rather than board header files.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/arch/avr32/config.mk b/arch/avr32/config.mk
index 4ab4745..b9b9631 100644
--- a/arch/avr32/config.mk
+++ b/arch/avr32/config.mk
@@ -6,7 +6,7 @@
 #
 
 CROSS_COMPILE ?= avr32-linux-
-
+PLATFORM_CPPFLAGS += -DCONFIG_AVR32
 CONFIG_STANDALONE_LOAD_ADDR ?= 0x00000000
 
 PLATFORM_RELFLAGS	+= -ffixed-r5 -fPIC -mno-init-got -mrelax