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>
diff --git a/tools/env/README b/tools/env/README
index 1020b57..24e31bc 100644
--- a/tools/env/README
+++ b/tools/env/README
@@ -2,11 +2,10 @@
 This is a demo implementation of a Linux command line tool to access
 the U-Boot's environment variables.
 
-In the current version, there is an issue in cross-compilation.
 In order to cross-compile fw_printenv, run
-    make HOSTCC=<your CC cross-compiler> env
+    make CROSS_COMPILE=<your cross-compiler prefix> env
 in the root directory of the U-Boot distribution. For example,
-    make HOSTCC=arm-linux-gcc env
+    make CROSS_COMPILE=arm-linux- env
 
 For the run-time utility configuration uncomment the line
 #define CONFIG_FILE  "/etc/fw_env.config"