am335x: configs: Fix usb ether boot support

am335x: configs: Fix usb ether boot support

CONFIG_SPL_NET_VCI_STRING is available only with BOOTP. So if
CMD_DHCP is enabled for SPL in usb ether boot, it will not pass
the right vendor name and failing to download the right file.
Also all the net CMD_* are not required in SPL builds. So defining
these only for non-SPL builds.

Reported-by: Yan Liu <yan-liu@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>