build:arm: Remove setting of CROSS_COMPILE environment variable

After Kbuild introduction, the CROSS_COMPILE environment variable has been
set to some default value (prefix arm-linux-).

This shall be removed since it breaks building u-boot for native arm target
(like qemu ARM).
Moreover not all compilers have arm-linux- prefix.

Additionally the u-boot cross compiles with CROSS_COMPILE= set explicitly-
e.g.:
CROSS_COMPILE=/ .... /arm-v7a-linux-gnueabi- make

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>