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= env

Going forward we can cross compile it by specifying CROSS_COMPILE:
make CROSS_COMPILE= env
This looks more natural.

Signed-off-by: Masahiro Yamada
Tested-by: Gerhard Sittig