top: Rename back CROSS_COMPILE_PREFIX to CROSS_COMPILE

The name CROSS_COMPILE is an industry standard used across
open-source project to specify cross-compiler prefix.

In fact, distro build systems such as buildroot and yocto
assume CROSS_COMPILE to be supported by projects added
as packages to these build systems.

This patch rename back CROSS_COMPILE_PREFIX to CROSS_COMPILE.

Signed-off-by: Anup Patel <anup.patel@wdc.com>