tools/env: cross-compile fw_printenv without setting HOSTCC

fw_printenv is a program which mostly runs on the target Linux.

Before switching to Kbuild, we needed to set HOSTCC at the
command line like this:
    make HOSTCC=<your CC cross-compiler> env

Going forward we can cross compile it by specifying CROSS_COMPILE:
    make CROSS_COMPILE=<your cross-compiler prefix> env
This looks more natural.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Gerhard Sittig <gsi@denx.de>