tools/env: document current cross-compilation issues and workaround

Signed-off-by: Luca Ceresoli <luca.ceresoli@comelit.it>

tools/env/README

@@ -2,6 +2,12 @@
 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
+in the root directory of the U-Boot distribution. For example,
+    make HOSTCC=arm-linux-gcc env
+
 For the run-time utiltity configuration uncomment the line
 #define CONFIG_FILE  "/etc/fw_env.config"
 in fw_env.h.