[AVR32] Use correct config symbol when setting cpuflags

As Robert P. J. Day pointed out, the CONFIG_CPU_AT32AP7000 symbol
wasn't used for anything. It should have been used to select the
correct -mcpu= options for CFLAGS.

-mcpu=ap7000 is the default anyway, so this patch shouldn't really
make any difference, but it's always nice to do things correctly.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>