net: Increase the size of the net_boot_file_name buffer

The net_boot_file_name buffer is used as storage for the bootfilename
command line argument to network boot commands such as tftp and nfs.

Increase the size of this buffer to 1024 bytes as the current size of
128 bytes is restrictive for arbitrary paths on the server.

Signed-off-by: Jacob Stiffler <j-stiffler@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>