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 new barebox logo to the tree ...

This adds the new barebox logo to the tree. This is added as svg
image which is converted to different png images during build time. The logo
can be found under /logo/barebox-logo-<width>.png in the running barebox.

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