net: Read bootfile from env on netboot_common()

net: Read bootfile from env on netboot_common()

Instead of depending on a env callback for bootfile, read it explicitly.

We do this because the bootfile can be specified on the command line and
if it is, we will overwrite the internal variable. If a netboot_common()
is called again with no bootfile parameter, we want to use the one in
the environment.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>