string: Fix (v)asprintf prototypes ...

Our asprintf and vasprintf have different prototypes than the glibc
functions. This causes trouble when we want to share barebox code
with userspace code. Change the prototypes for (v)asprintf to match
the glibc prototypes. Since the current (v)asprintf are convenient
to use change the existing functions to b(v)asprintf.

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

Add defaultenv command ...

This adds a command to explicitly restore the environment (or
parts thereof) from the default environment.

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