At the moment globalvar code tries to use nv_device
even if nv_device is not registered.

How to reproduce the problem:

    barebox$ make sandbox_defconfig
    ...
    barebox$ sed -i "s/\(CONFIG_NVVAR\)=y/# \1 is not set/" .config
    barebox$ sed -i "s/\(CONFIG_DEFAULT_ENVIRONMENT_GENERIC_NEW\)=y/# \1 is not set\n# CONFIG_DEFAULT_ENVIRONMENT_GENERIC is not set/" .config
    barebox$ make oldconfig
    ...
    barebox$ make
    ...
    barebox$ ./barebox
    Segmentation fault

This patch blocks nv_device use if CONFIG_NVVAR is not set.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>