magicvar: Add magicvar macro with additional name argument ...

Currently magicvar fails on variables containing a '.' because we can't
use these as C names. Overcome this by adding a new macro which allows
to specify a name seperately.

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

add magicvar command ...

The magicvar command gives an overview about all environment variables
with a special meaning.

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