buildroot: copy build env to rootfs $HOME directory

few test scripts requires platform and flavour information to run
e.g. test_mpmm.sh, adding build env to rootfs will be useful for such
scripts which can parse platform/flavour configuration directly from
build_env.cfg file instead of providing configuration to them on command
line.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>