Makefile: only check the CROSS_COMPILE when compile at91bootstrap

When running a 'make clean', 'make xxx_defconfig', 'make mrproper'
and etc, it will report an error:
   Environment variable "CROSS_COMPILE" must be defined!

Actually, above commands no need to define CROSS_COMPILE.
So move this check only when compile the binaries.

Reported-by: Zhang Ke <ke.zhang@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Makefile: check hostarch is arm, if yes no need to check CROSS_COMPILE

Tested-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>