AVR32: Build position-independent u-boot

AVR32: Build position-independent u-boot

Add -fPIC -mno-init-got to the avr32-specific CFLAGS to make u-boot
position independent. This will make relocation a lot easier.

-mno-init-got means that gcc shouldn't emit code to load the GOT
address into r6 in every function prologue. We do it once and for
all in the early startup assembly code, so enabling this option
makes u-boot a bit faster and smaller.

The assembly parts have always been position-independent, so no code
changes should be necessary.

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