fw_env.c: Switch get_config to use '%ms' in sscanf

We currently limit ourself to 16 characters for the device name to read
the environment from.  This is insufficient for /dev/mmcblk0boot1 to
work for example.  Switch to '%ms' which gives us a dynamically
allocated buffer instead.  We're short lived enough to not bother
free()ing the buffer.

Signed-off-by: Tom Rini <trini@ti.com>