net: Prefer command line arguments

We can call commands like dhcp and bootp without arguments or with
explicit command line arguments that really should tell the code where
to look for files instead.

Unfortunately, the current code simply overwrites command line arguments
in the dhcp case with dhcp values.

This patch allows the code to preserve the command line values if they
were set on the command line. That way the semantics are slightly more
intuitive.

The reason this patch does that by introducing a new variable is that we
can not rely on net_boot_file_name[0] being unset, as today it's
completely legal to call "dhcp" and afterwards run "tftp" and expect the
latter to repeat the same query as before. I would prefer not to break
that behavior in case anyone relies on it.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>