uboot-phytec

Makefile: clean boot.bin

When build for Atmel related boards which support SPL,
it will generate boot.bin, also clean when it when do
"make clean" operation.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>