When a build with a compressed default environment is done and then
environment compression is disabled, barebox_default_env.h is not
regenerated because its dependency (now barebox_default_env) is still
up to date. This results in a corrupt default environment.

This patch fixes this by adding a cmd_env_h and changing barebox_default_env.h
to FORCE. As a side effect barebox_default_env.h is no longer generated in
include/generated/ but in common/ as we have to add barebox_default_env.h to
$(targets) (extra-y).

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>